Onix is available in the Union Cave, Victory Road, Rock Tunnel, and Mt. Silver. You can also trade a Bellsprout for an Onix with an NPC in Violet City.
DISCLAIMER: It is possible to legally evolve Onix into Steelix BEFORE the Elite Four by going into the Rocket Hideout in Mahogany, getting the Thief TM, and then using it to steal a Metal Coat from a wild Magnemite [8% chance] in Route 38 or 39. Further Metal Coats may only be obtained on the S. S. Aqua after rescuing the little girl, or by trading a Dugtrio for Maggie the Magneton in the Power Plant (the item is attached to the traded Pokémon).
From the early stages of Johto to the late stages of Mt. Silver, Onix is one of the most valuable Pokémon you can ever use in GSC. In the early-game, Onix is essentially a poor man's Geodude. While it has an amazingly high 160 base Defense stat and a pretty solid 70 base Speed stat, its terrible base 45 Attack prevents it from dealing significant damage to the opposing Pokémon which Geodude, by contrast, can easily 2HKO at worst. Also, for STAB options, Onix is forced to use either Rock Throw or Dig, both of which are rather weak attacks even after a Screech or two. Nonetheless, the one major advantage Onix has over Geodude is that fact that it evolves into Steelix, which depending upon when you do that can be game-changing. Like seriously. You can have Steelix by Morty. Onix can still take on the vast majority of the early-game all by itself, albeit not as easily or reliably as Geodude can.
A good reason to use Onix over Geodude is that you can receive Onix by an in-game trade in which you trade a Bellsprout to an NPC in Violet City. This will enable Onix to receive 1.5x the amount of experience that it normally would, a fact that can be pretty significant if you are doing a limited-grinding run. However, keep in mind that in Generation II, a traded Pokémon above Level 10 will NOT be fully obedient to you until after you get your second badge. If, because of this fact, you would rather catch Onix in the wild, you can do so in the Union Cave. Overall, although its Attack stat is terrible, Onix is a very good, high-tier Pokémon to use in GSC.
Gym #1 - Falkner (Violet City, Flying-type): If you have an Onix by this point, that means that you obtained it via trade. This means that if you were to try to grind it to level 14, it would not be obedient to you. Thus, it is ultimately more efficient to challenge Falkner at about level 9. The battle will take awhile, and you will probably have to switch out Onix a couple of times to offset the Mud Slap accuracy loss, but there is really no way you can lose as long as you don't try anything stupid. Screech is very useful for this fight.
Gym #2 - Bugsy (Azalea Town, Bug-type): Remember the phrase "there is no danger of dying" in the previous spot? Well, that is no longer applicable. Because of Onix's horrendously low Attack, a single Rock Throw probably will NOT be able to KO the Scyther. This means that if you received your Onix via trade and it happens to be disobedient, then there is a legitimate possibility of being Fury Cutted to death. If, however, you caught your Onix in Union Cave, then the odds of winning this fight go up drastically. All you really need to worry about is if Rock Throw happens to miss several times in a row, which is very unlikely.
Rival (Azalea Town): From this point on, assuming you have already obtained the Hive Badge, you no longer need to worry about whether or not your Onix will obey you or not; it always will. Because of Onix's low Attack, it will be unable to OHKO either the Gastly or the Zubat, and you will be prone to their respective status moves. Against the starters, Onix autowins against Cyndaquil (barring major hax such as Ember burn) and autoloses against Croconaw and Bayleef. This will be the theme for the rest of the run.
Gym #3 - Whitney (Goldenrod City, Normal-type): You probably should not risk it with the Clefairy due to the unpredictability of Metronome (it may select a Water- or Grass-type attack), but you can beat the Miltank with relative ease by using Screech x3 + Rock Throw. Dig, while not necessary, can be very useful as it will force Miltank's Rollout counter to reset.
Rival (Burned Tower): Again, Screech/Dig/Rock Throw is the way to go, but be wary of hax/residual damage.
Gym #4 - Morty (Ecruteak City, Ghost-type):
Onix: Although Onix can use STAB Dig in a Levitate-less generation to inflict STAB supereffective damage on each of his Pokémon, Onix will still probably fail to solo this fight. Spite will hack away at Dig's low 10 PP, Curse will still work against Onix even if it is underground, and Gengar's Hypnosis + Dream Eater will destroy Onix. Thankfully, though, in this generation Shadow Ball is a physical move, meaning that Onix can easily tank several from Gengar off its very low Atk stat. Equipping the Mint Berry is definitely recommended.
Steelix: The similar principles for the Onix still apply here, with some notable exceptions. The most important factor is that Steelix has considerably higher special bulk, as well as a Steel-typing, meaning that it can tank a few Dream Eaters from Gengar if need be. While it will probably be able to OHKO the Gastly and both Haunters with Dig, Steelix's terrible base 30 speed will guarantee that these ghosts will outspeed and do crazy sleep stuff.
Eusine (Cianwood City, Crystal only):
Onix: His Drowzee is definitely a bad matchup because it knows both Confusion and Hypnosis + Dream Eater. You should be able to take on the Haunter, but watch out for the potentially deadly Mean Look + Hypnosis + Curse combo. The Electrode should be pretty easy but Sonicboom will do a good chunk of damage due to Onix's low HP stat.
Steelix: Steelix has no problems with this fight, being either resistant or immune to every single one of Eusine's attacks. Just be wary of Curse traps, as usual.
Gym #5 - Chuck (Cianwood City, Fighting-type):
Onix: You can take on the Primeape if you really want to, but it is not advised. Its STAB supereffective Karate Chops will outdamage your weak Digs. Poliwrath is a definite no-no as it will easily OHKO you with Surf.
Steelix: Primeape should not be very hard to take out at all despite your type weakness. Steelix can easily tank a handful of unboosted Karate Chops and 2/3HKO with either Dig or Iron Tail. Unfortunately, Steelix does not match up very well against the Poliwrath at all as it has STAB supereffective Surf while being able to shrug off many Digs.
Gym #6 - Jasmine (Olivine City, Steel-type):
Onix: You can easily outspeed and OHKO both of her Magnemites with Dig. The opposing Steelix, however, is a bad matchup. If you are in the mid 30s, you can probably avoid the OHKO from the Iron Tail, but even a STAB supereffective Dig will not do any significant damage in return. Let a special attacker handle this matchup if at all possible.
Steelix: Steelix can easily solo this fight. While you may not be able to outspeed the Magnemites, the worst they can do is annoy you with Supersonic and deal chip damage with Sonicboom. Against the opposing Steelix, you win easily as long as you're packing Dig. It will not be able to deal any sort of damage in return while you can spam Screech + Dig.
Rocket Executive battle #1 (Team Rocket HQ):
Onix: Onix may get a little annoyed from time to time during this fight, but since his team is pretty underleveled, you should be fine. Zubat's Confuse Ray may be annoying, and Koffing may end up poisoning you, but that's pretty much it.
Steelix: Your crucial immunity to poison comes into play here. Basically the only thing that kind slightly throw you off is Zubat's Confuse Ray; otherwise, spam your strong physical move of choice (Iron Tail, Dig, Rock Throw, Strength, etc) for the win.
Rocket Executive battle #2 (Team Rocket HQ):
Onix: Her Arbok can deal some chip damage with Bite, especially if she gets a flinch or two, but it's nothing to worry about. Her Gloom, on the other hand, can deal significant damage to you with Absorb, if not OHKO you outright. Stay away. Also, if for whatever reason you find yourself at any lower than 1/3 of your health by the time Murkrow comes in, be very careful of its ability to Pursuit you away.
Steelix: There is absolutely nothing you need to worry about here. The worst thing that she can do against you is put you to sleep with Gloom and then try to chip away at your health with Absorb, but you have a Steelix. Just mow 'em all down with whatever physical move you feel like.
Gym #7 - Pryce (Mahogany Town, Ice-type):
Onix: Pryce is one of Onix's very few "really bad matchups." All three of his 'mons have access to decently powerful Ice moves, all of which can deal heavy damage to Onix. Additionally, both his Seel and Dewgong have Rest, and his Poliswine is pretty bulky in general, so there is no way Onix is going to be doing anything useful in return except MAYBE throw off a Screech and then retreat. But really, why would you risk losing such an amazingly useful 'mon in Onix?
Steelix: With pretty respectable special bulk and powerful offensive moves, Steelix has a substantially easier time in this fight than its younger sibling. Against Seel and Dewgong, using Screech + Dig/Strength/Iron Tail is probably the best way to go. Just keep in mind that both of them have Rest for reliable recovery and Aurora Beam, whose 10% chance of lowering attack can be very annoying along with its decent damage. Finally, Poliswine may in fact be OHKO'd by a single Iron Tail if you get lucky. Even if you don't, Poliswine will really struggle to deal any significant damage back to you, even with Blizzard.
Rocket Executive battle #3 (Goldenrod Radio Tower):
Onix: Simply alternate between Screech and Dig. Unless you are in the mid 30s, you may not be able to OHKO the Koffings with Dig even after using Screech once. Also watch out for poison. Although Onix can easily tank a couple of SelfDestructs from Koffing, just don't let your health go down too far, because that Weezing's Explosion is not something you can shrug off when in the red.
Steelix: Thanks to its titanic bulk immunity to poison, and pretty decent Attack, Steelix has virtually no problems here whatsoever. Depending upon your level and stat experience, a single Dig may or may not be able to OHKO the Koffings, but ultimately they won't be able to do any significant damage to you in return.
Rival (Goldenrod Underground):
Onix: As usual, Onix fares pretty well against Golbat, Haunter and Magnemite; just watch out for their respective status moves. His new acquision in Sneasel does not have any Ice-type moves, so if you want Onix to handle that too, it probably can. As for the starters, Quilava is an easy matchup for Onix provided you don't get burn haxxed by Flame Wheel. Feraligatr and Meganium, however, are still definite no-no's.
Steelix: Steelix can easily 2HKO at worst the first four 'mons with its dual STABs, only fearing Golbat's Confuse Ray and Haunter's Curse. Against the starters, Steelix unfortunately does not do super well: Quilava and Feraligatr will wear it down considerably with their STAB supereffective attacks in Flame Wheel and Water Gun; Meganium, meanwhile, is very bulky and can outstall with Reflect and chip away with Razor Leaf.
Rocket Executive battle #4 (Goldenrod Radio Tower):
Onix: All this guy has is a level 36 Golbat, so simply use Screech once or twice and then Rock Throw away. Just be wary of the annoying Confuse Ray + Bite combo.
Steelix: Screech + Iron Tail should be enough to OHKO this Golbat, while the only thing it can do back is annoy with Confuse Ray.
Rocket Executive battle #5 (Goldenrod Radio Tower):
Onix: Onix has supereffective coverage against all three of his 'mons, but can get worn down easily. Houndor can chip away with Faint Attack, but your Dig will do a great deal of damage if not OHKO outright. The same is true for Koffing, although it can be very annoying with Smokescreen and potential Sludge poison hax. The Houndoom is by far the most threatening 'mon and will probably outspeed and 2HKO with Faint Attack, while you may not be able to 2HKO with Dig.
Steelix: Despite having a Fire weakness, Steelix arguably does better here than its younger sibling. For one, Steelix easily OHKO's Houndor and potentially OHKO's Koffing. Houndoom is a little trickier, but still manageable: its Ember will be an easy 3HKO on Steelix at worst, but Steelix can definitely 2HKO back with Dig. It should be noted that Houndoom does roughly the same amount of damage to a Steelix with a supereffective Ember than it does against Onix with a neutral Faint Attack.
Suicune (Bell Tower, Crystal only):
Onix: There is no way in the world for Onix to take a BubbleBeam to the face, so it should just stay away from this battle.
Steelix: Steelix is bad against Suicune, but it has decent special bulk. Unless Suicune puts up Rain Dance, Steelix can hope to deal some damage with Dig. However, Dig also carries the risk of giving Suicune a turn to set up the weather and then hit harder. It would be wise to avoid using Steelix here.
Gym #8 - Clair (Blackthorn City, Dragon-type):
Onix: Her lead Dragonair has Surf, so obviously avoid that matchup. Then, she has two more Dragonair: one that knows Thunderbolt, and another than knows Ice Beam. It is theoretically impossible to determine which one is which until it actually uses one move or the other; however, one could conceivably manipulate the AI to send out a particular one. The Kingdra should be avoided at all costs as it will swiftly OHKO you with Surf.
Steelix: Steelix does a somewhat better job here than its younger brother. Although the lead Dragonair will probably fail to OHKO Steelix with Surf, it will still likely 2HKO, so the best thing you can do is use Screech and then retreat. Against the Dragonair with Thunderbolt, you are resistant or immune to every single one of its moves, so you can easily beat it with the usual Screech + Dig/Iron Tail. he other Dragonair will attempt to chip away at your health with Ice Beam, but Steelix will be able to shrug off a handful of Ice Beams and deal heavy damage with Iron Tail. Kingdra is still a bad matchup because it will wash you away with Surf, so you should let a bulky Water- or Grass-type handle that matchup.
Rival (Victory Road):
Onix: This fight is rather similar to the previous Rival fight: use the appropriate STAB attack on Golbat, Magneton, Sneasel, Haunter and Typhlosion, and avoid Feraligatr and Meganium. The only difference here is that Rival has added a Kadabra, against which Onix has a bad matchup. Let a fast physical attacker or bulky special wall handle that.
Steelix: Steelix can take out Golbat, Magneton, Sneasel, Haunter and Kadabra without too much trouble at all, although take note that all of them will likely outspeed you. Typhlosion and Feraligatr are bad matchups and thus should be avoided. Meganium is the usual stall fest.
Elite Four Will (Indigo Plateau, Psychic-type):
Onix: Since every single one of Will's Pokémon is packing STAB Psychic, Onix really struggles here and will likely get 2HKO'd at worst in every single matchup. The best thing Onix can do is use Screech and then retreat away; sadly, none of Onix's unboosted attacks will deal any sort of significant damage to Will's 'mons before it's too late.
Steelix: Steelix performs remarkably well in this fight, resisting quite well the Psychics that Will loves to use. Against the two Xatus, the worst thing you have to worry about is Confuse Ray hax, but thankfully you won't do a lot of damage to yourself in confusion. Exeggutor can be rather tricky because it has both Reflect and Leech Seed, meaning that you may want to switch your Steelix out after getting off a couple of Screeches. Slowbro's only attacking moves and Body Slam and Psychic, giving you plenty of time to use two or three Screeches and then spam Earthquake to your heart's content. Jynx is definitely the toughest matchup as it can put Steelix to sleep with Sleep Powder and deal decent damage with her specially-based Ice Punch. If you manage to shrugg off sleep, however, then Iron Tail is an easy OHKO. In general, while Steelix resists Psychic, you definitely don't want to have to tank several in a row, especially if they get a Special Defense drop.
Elite Four Koga (Indigo Plateau, Poison-type):
Onix: Ariados has Giga Drain, so it will definitely beat you one-on-one. The Venemoth can wittle away at your health with Psychic, although you have the potential to outspeed and deal decent damage with Earthquake. The Forretress can chip away at your health with Swift and then shave off give or take half your health with Explosion, so be very careful about your HP management. Muk is plenty bulky enough to tank a couple of unboosted Earthquakes and harass you with Toxic, Acid Armor and Minimize. Crobat, similarly, can tank a handful of Rock Throws and has both Double Team and Toxic, so if you don't play your cards right, you may find yourself on the losing end of a stall war.
Steelix: Virtually the only thing preventing Steelix from clean sweeping this fight is accuracy hax. Other than that, just select the appropriate STAB attack, using Screech when necessary, and GG Koga.
Elite Four Bruno (Indigo Plateau, Fighting-type):
Onix: You can beat the Hitmontop just fine by spamming Earthquake as it uses Dig (the damage output is actually doubled for that turn). The opposing Onix matchup probably just comes down to which is higher leveled, but hopefully you have a 'mon with Surf for this fight. You should be able to take on the Hitmonchan as its Mach Punch won't be doing a whole lot to you, but there are plenty of other mons who can handle Hitmonchan better. Definitely stay away from Hitmonlee and Machamp, as while you may be able to tank a hit from full health, you really won't be able to do a whole lot in return.
Steelix: Since the Hitmontop will always Dig first turn, you can use Earthquake to go for the OHKO. You can take on the Onix decently enough, but a Water or Grass-type handles that matchup much better. Hitmonchan will do pathetic damage to you while you will likely 2HKO in return with Earthquake. Hitmonlee is a little trickier as its Hi Jump Kick will do decent damge to you, but you can still tank at least one hit and then retaliate with Earthquake. Machamp not only has more bulk and Attack than Hitmonlee, but it also carries a high critical-move in Cross Chop, a critical may in fact KO Steelix.
Elite Four Karen (Indigo Plateau, Dark-type):
Onix: Umbreon is bulky as all hell, so you'll have to use Screech three times before you can deal any significant damage to it, which will be difficult as it will probably be spamming Sand Attack or use Confuse Ray like a troll. Vileplume is a definite no-no. You may be able to OHKO the Gengar with Earthquake, but watch out for Destiny Bond. It almost definitely won't use Dbond first-turn, though. Murkrow isn't that bad, but you are probably going to want to be at full health to take on the Houndoom, which you should be able to tank a hit from and defeat with Earthquake. It may be a damage range depending on your level and stat experience, though.
Steelix: Umbreon is super bulky and will require multiple Screeches and Earthquakes to take down. Thankfully it can't do any significant damage back to you, but it can use Confuse Ray and Sand Attack. Vileplume can be handled decently enough, but since it has STAB Petal Dance, you probably want to use either a special wall or a Grass-resist. Both Murkrow and Gengar cannot do any sort of significant damage to you, so you are free to destroy them with the appropriate STAB attack. Even though Gengar has Destiny Bond, it will never use it first turn, so you're good there. Houndoom is a very difficult matchup: while you can most likely tank a Flamethrower from full health and OHKO with Earthquake, it's up to you whether you want to risk the potential crit or Burn chance.
Champion Lance (Indigo Plateau, Flying-type):
Onix: Supereffective Rock Throw unfortunately will barely dent his Dragonites, although Onix matches up well with his Aerodactyl and Charizard. You can beat both without problems, just stay away from the rest.
Steelix: Not a very fortunate matchup, as Gyarados can use the Rain Dance + Surf combo, while Charizard has STAB Flamethrower, and the ace Dragonite knows Fire Blast. On the upside, Steelix resists every single hit the other three have, minus Blizzard. It should be able to pull off an easy victory against Aerodactyl with Iron Tail, and then stall out the other two.
From this point onwards, you can fight the gyms in any order, though you will need to retrieve the Machine Parts from the Cerulean City gym before you have access to the earlier portion of Kanto. Feel free to anticipate or postpone any battles as needed.
These matchups assume that Onix has evolved into Steelix, exception made for Blaine.
Gym #9 - Brock (Pewter City, Rock-type): Spamming Iron Tail or Earthquake against the Rock/Ground-types. Skip the fossils unless you are very overleveled, or desperate; both of them have STAB Surf, which will hurt a lot coming off Omastar's Special Attack, but will also be less troublesome from Kabutops.
Rival (Mt. Moon, optional): As usual, Steelix's deal is everything but Typhlosion and Feraligatr. Alakazam and Gengar are evolved now, but not much of their threat levels has changed. Steelix can still KO almost everything with Earthquake, but be careful if its health errs on the low side.
Gym #10 - Misty (Cerulean City, Water-type): This gym is bad for Steelix, on all fronts. However, if you postpone it until after the others, you can gain a good level advantage and spam Earthquake against Misty. Her Pokémon have well-rounded, but average stats in general, with Starmie being the more offensively oriented. Still, use alternatives if you have them.
Gym #11 - Lt. Surge (Vermilion City, Electric-type): Steelix laughs at this gym. Spam Earthquake and win.
Gym #12 - Erika (Celadon City, Grass-type): Steelix's acquired neutrality to Grass is handy here. Iron Tail will serve you well in this gym. Bear in mind you will be outsped, and SolarBeam is still not really a weak move, but the level difference should compensate for that.
Gym #13 - Janine (Fuchsia City, Poison-type): Again, a gym weak to Ground, and at ridiculous levels to boot. Spam Earthquake.
Gym #14 - Sabrina (Saffron City, Psychic-type): Steelix's resistance to Psychic is helpful, but don't overestimate its low Special Defense. If you manage to land an Earthquake on Mr. Mime and Alakazam, they may be OHKOed depending on your level, though Espeon is very likely to survive a move. In addition, some of her Pokémon have Reflect, which will make Steelix's job harder. Steelix can be helpful here, but be sure to bring alternatives along.
Gym #15 - Blaine (Seafoam Islands, Fire-type):
Onix: If you've kept your Onix an Onix for so long, this is time for it to compensate your efforts. Onix can easily solo this gym with Earthquake. Beware, though, as Rapidash's Fire Blast is still not a joke.
Steelix: This is possibly the only matchup in which Steelix fares worse than Onix does. Fire attacks coming off these Pokémon are very strong, and nothing to sneeze at. Steelix's low Speed also means it will likely not land a hit on anything but Magcargo, which will definitely go down to Earthquake. Avoid the rest.
Gym #16 - Blue (Viridian City): Great matchup against Pidgeot, which can't hurt Steelix in the slightest. Exeggutor is also at a considerable disadvantage, and should be offed by an Iron Tail or two. Rhydon can be beaten as well, but if it crits, Steelix may suffer serious damage; consider this. Alakazam is much like Sabrina's; can be attempted, but don't rely on Steelix too much as it might not make it alone. Arcanine and Gyarados have supereffective STAB to hit Steelix with, so avoid them.
Rival (Indigo Plateau, optional): This battle is essentially identical to the previous. Like before, Typhlosion and Feraligatr are not good opponents for Steelix, as they have supereffective STAB to hit it with. Feel free to beat everything else, though.
Red (Mt. Silver): You could set up Curse against Pikachu, but it would likely not work much. Charizard and Blastoise, Steelix's bigger enemies in this team, will make Steelix need to duck in the Ball as soon as they come out. Steelix can, however, defeat everything else almost effortlessly, hard-walling the dangerous Snorlax and providing a useful resistance to Espeon's Psychic as well as immunity to Venusaur's Poison moves.
Onix starts with Tackle and Screech. Both Onix and Steelix learn the same level-up moves, save for the last one, which is unique to Steelix. Screech will be surprisingly useful for a while, as Onix's Attack will soon turn out to be abysmal compared to that of its enemies. At level 10, it gets Bind, which is nothing special. Much better is Rock Throw, learned at level 14, which will be its main STAB for a while. At level 23 it learns Harden, which it doesn't really need with its beast Defense. It also gets Rage at level 27, which is terrible. At level 36, it learns Sandstorm, which is worth a slot for Toxic carriers, but will not boost its Special Defense in this generation. Then there is Slam at level 40, absolutely useless as you will have access to Strength much sooner. Steelix is the only one that can learn Crunch, which it gets at level 49, but given its bad Special Attack, you won't be missing out if you don't get it.
TMs make Steelix's life much easier. There are the obvious options Earthquake and Iron Tail, as well as Dig before the Earthquake TM comes around. In the postgame, Curse gives Steelix a great boost as well. Toxic is also worth mentioning in combination with Sandstorm, for a more defensive set.
Recommended moveset: Earthquake, Iron Tail, and any two between Curse / Sandstorm / Swagger / Toxic
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
Onix Line Ratings
At what point in the game should I be evolved? If you plan on waiting until Kanto, beat Blaine first and evolve then. If not, it would be a good idea to get a Steelix as early as possible, by using Thief on the wild Magnemite.
How good is the Onix line in a Nuzlocke? Although it is more or less outclassed by Geodude, Onix is still an incredible Pokémon which will sponge up a myriad of otherwise dangerous attacks, both early-game as an Onix and late-game as a Steelix. Onix's claim to fame as one of the only true counter's to Whitney's Miltank is reason enough to justify a team slot. If you want a super solid physical tank but are tired of using Geodude all the time, then look no further than Onix.
Onix's type matchups:
Weaknesses: Ground, Steel, Ice, Fighting, Grass (x4), Water (x4)
Resistances: Flying, Normal, Fire, Rock, Poison (x0.25)