Oddish is encountered on Route 5, 6, 24, 25, Ilex Forest and the Johto Safari Zone at night only.
The Sun Stone can be acquired on Route 13 (gift from Camper Tanner), as a first prize in the Bug-Catching Contest, in the Pokeathlon Dome post-game or as an item held by wild Solrock.
Ah, Bellosom. The "assumed" worse Evolution of the Oddish line. However, it is actually quite tanky, as the typing provides more resistance later on than Vileplume and works somewhat nice as a physical attacker. The little cutie can deal well with Bruno, Whitney and her murderous moo-moo and the Kanto part. All in all, it can take you far.
Proton (Slowpoke Wells): Zubat is pathetic, but beware of Confusion hax. Koffing can hit you with Smog and Smokescreen and can poison, but... it's Level 12, so if Oddish is sufficiently leveled, you can stall them to death. Still not recommended though. (Suggested Level: 15)
Gym #2 - Bugsy (Azalea Town, Bug-type): No. U-turn from Scyther can 2-3HKO Oddish, and Acid is 6HKO. It can take on the pods, but freaking everything can. The best it can do is Sleep Powder the Scyther and leave the rest to your Geodude or whatever. (Suggested Level: 19)
Rival (Azalea Town): Not really. Gasty can Curse trap, Zubat has Bite and Supersonic, be beware of Croconaw due to to Bite and stay very far away from Quilava. Bayleef is a good matchup though, due to Acid and Bayleef cannot hit Oddish for anything. (Suggested Level: 20)
Gym #3 - Whitney (Goldenrod City, Normal-type): Is it still a Oddish/Gloom or a Bellossom without Magical Leaf? If yes, take it very, very far away from Miltank. Stomp 3/4HKOs it and it has a Lum Berry to laugh at Sleep Powder. With Magical Leaf though, in order to Miltank to off Bellossom, it would have to pull a lot of amazing bullshit, and Bellossom can decapitate Miltank with the aforementioned move in 2 hits, but still, be careful. Have Reflect and Sleep Powder up first though, because you never know what will happen. Clefairy is... just put up a Substitute and kill it. (Suggested Level: 23)
Rival (Burned Tower): Same verse, same as first but Magnemite is also bad due to Sonicboom. (Suggested Level: 23)
Gym #4 - Morty (Ecruteak City, Ghost-type): Bad matchup for Bellossom here, due to it's inability to hit anything for actual damage. Three fourths of Morty's Pokemon can use Curse, and Gengar... no, just no. Use something that can damage Morty's Ghost-types and leave Bellossom out for the battle. If you absolutely have to, Sleep Powder the Gengar and switch out to something else. You may be able to off his Gastly, but, again, freaking everything can. (Suggested Level: 25)
Eusine (Cianwood City): Not encouraged. Drowzee is safe, due to Dream Eater only achieving a 5KHO, while Bellossom's Magical Leaf will 4HKO at worst and 3HKO at best. Keep an Awakening or two though. Haunter... see Morty. Electrode can be dangerous. Thunder deals as much damage as Drowzee's Dream Eater, and Sonicboom is the same. Keep some healing items handy and you will be fine. No, the real danger is Screech+Rollout, as it can kill Bellossom in 4 consecutive hits. However, giving Bellossom Protect and keep healing items on your hand should be solid to take Eusine on. (Suggested Level: 32)
Gym #5 - Chuck (Cianwood City, Fighting-type): Meh. Primeape's Rock Slide is pitifully weak, scoring a meager 5HKO, while Bellossom's Magical Leaf 3HKOs Primeape, and it can't pull some amazing bullshit with Focus Punch, since Magical Leaf ignores Double Team. Poliwrath, however, should not be fought unless you are extremely overleveled, as Focus Punch 2HKOs Bellossom and OHKO with a Critical Hit, but Bellossom's Magical Leaf only 3HKOs Poliwrath. If you absolutely need to, give Bellossom a Chesto Berry and pray that Victini is on your side. (Suggested Level: 34)
Gym #6 - Jasmine (Olivine City, Steel-type): Solid. The Magnemites resist most of Bellossom's move, except for Drain Punch, which can be obtained on the route above the city, and said move score a 2HKO, while those Magnemites can't do jack shit aside from Confusion hax Bellossom. Steelix can kill Bellossom with 2 Iron Tails at -2 Defense, but, the thing is, unless it has 0IV 0EV -Speed Nature, Bellossom will outspeed the iron snake and 3HKO Steelix. Just heal Bellossom first and hope Steelix doesn't achieve a Critical Hit and you will be fine. (Suggested Level: 35)
Archer (Team Rocket HQ): Zubat is Level 22. Your Bellossom is at least 10 levels higher. Magical Leaf will 4-5HKO and the worst Zubat can do is Confuse Ray Bellossom. Koffing's Selfdestruct will 3HKO, and it can use that once only. Keep Bellossom at high health and you'll be fine, as Magical Leaf 2HKOs Koffing. Smog is pathetic and so is Raticate. (Suggested Level: 35)
Ariana (Team Rocket HQ, tag battle with Lance): Arbok is sad, with Crunch 6HKOing Bellossom and Magical Leaf 5HKO back. Gloom is unsafe, due to Acid's 5HKO compared to Magical Leaf's 8HKO. Murkrow is even worse, because Wing Attack 3HKOing Bellossom and Magical Leaf 4HKOing Bellossom. But you have a Level 40 Dragonair as a partner, so heal up and reap free Experience Points. If you have Return, this will be a breeze (Suggested Level: 35)
Gym #7 - Pryce (Mahogany Town, Ice-type): Seel is safe: Magical Leaf 2HKOs it and Icy Wind can only manage a 5HKO after Hail. Dewgong is, as Magical Leaf achieves the same damage, and it can Aurora Beam you for a 3HKO. For the first two, Rest can be annoying, but manageable. If you want to deal with Piloswine, reset the Icy Wind Speed drop and cancel Hail first, as it has Blizzard. Both Blizzard and Magical Leaf 2HKO the receiving end of each move, but Bellossom is faster and Blizzard have horrid accuracy and Magical Leaf ignores Snow Cloak evasion hax bullshit. But Critical Hits and Freezing are a thing, so if you do need Bellossom to fight Piloswine, give it the Aspear Berry and hope RNGesus is nowhere near Pryce. If Hail is up, stay away. (Suggested Level: 36)
Petrel (Goldenrod Radio Tower): He has 6 exploding Koffings/Weezing. You can kill one, but, again, Critical Hits are a thing, so... don't. (Suggested Level: 37)
Rival (Goldenrod Underground): Nah. Bellossom should not fight Golbat, as the latter has Air Cutter, 4HKOing the former, while Bellossom can only get a 3HKO. And Golbat is faster. The Haunter is the same situation. If Bellossom has Drain Punch, the Magnemite and Sneasel are dead meat. The starters are dangerous now and should be avoided. Meganium has Poisonpowder and Synthesis to stall you out, Feraligatr has Ice Fang and Quilava... 'nuff said. (Suggested Level: 38)
Proton (Goldenrod Radio Tower): If Bellossom has Return, Golbat is easy to off: Wing Attack 4HKOs the former, while Return 2HKOs the latter. Weezing... nope. (Suggested Level: 38)
Ariana (Goldenrod Radio Tower): Return is a 3HKO, while Crunch from Arbork is 6HKO. Go nuts. Vileplume's Acid is a 4-5HKO, and Bellossom's Return is also a 3HKO, so be ready to heal. Murkrow is dangerous, so be careful - its Wing Attack 3HKOs Bellossom, and Return 2HKOs back, but Murkrow is faster. It's Pryce all over again. Better let something that isn't weak to Wing Attack go for it. (Suggested Level: 39)
Archer (Goldenrod Radio Tower): Houndour can be defeated with Drain Punch as long as it doesn't Flinchax you to death. Koffing has Sludge and Houndoom have Fire Fang. They both can 2-3HKO you, so no. (Suggested Level: 39)
Gym #8 - Clair (Blackthorn City, Dragon-type): Eh. Gyarados is not a good matchup for Bellossom, due to Dragon Rage and Return 4HKOing the former. The Dragonairs are safe to kill - Dragon Pulse only manages to 4-5HKO, Fire Blast is a 3HKO, and if Bellossom is holding a Silk Scarf, Return 2HKO them both. Kingdra is a no go - it's Critical Hyper Beam OHKOs Bellossom, whose Return is a 4HKO. Flowers are not meant to be obliterated, you know. (Suggested Level: 41)
Kimono Girls (Ecruteak Dance Theater): Quite well. A Bellossom with Drain Punch is enough to deal with Umbreon if the latter doesn't Flinchax/Confusehax it to death, with Drain Punch 4HKOing Umbreon and heal Bellossom itself in the process. Espeon shouldn't be fought as the situation is the same as Ariana's Murkrow. However, if you have either Leaf Blade from the Move Relearner or Petal Dance from Oddish (assume it survives that long), the situation is now akin to Pryce's Piloswine, as both moves 2HKO. Flareon's a no go. Jolteon is easy enough: It's best attack - Last Resort - 4HKOs Bellossom, and Magical Leaf 3HKOing Jolteon back. Magical Leaf is there because of Double Team shenanigans. Vaporeon is easy, as Aurora Beam 3-4HKO, while Leaf Blade has a chance to OHKO it outright, and Petal Dance 2HKOs it. (Suggested Level: 43)
Ho-Oh (Bell Tower, HeartGold only): No. It's a Legendary Pokemon with a STAB Super Effective move. (Suggested Level: 45)
Lugia (Whirl Islands, SoulSilver only): Same as above. (Suggested Level: 45)
Rival (Victory Road): Most of his team is now quite easy due to Sword Dance if you run a Physical set except for Typhlosion and Haunter (obviously). Boost to +6 and obliberate his team. If not, stay very far away. (Suggested Level: 47)
Elite Four Will (Indigo Plateau, Psychic-type): Quite well. His Xatus are safe to fight because of their best attacks only able to achieve a 4HKO, and at +2 Attack Return has a chance to OHKO. So is his Jynx, as Ice Punch 4HKOs and at +2 Leaf Blade OHKOs. Slowbro does pathetic damage with Psychic and +2 Leaf Blade 2HKOs it. You can beat Exeggutor but it is super annoying so let somebody else do it. If you need to, take some Awakenings. (Suggested Level: 50)
Elite Four Koga (Indigo Plateau, Poison-type): Not good. Taking on Ariados is unwise, due to both Bellossom's Return and Ariados's Poison Jab 2HKOing the opposing, but Bellossom is faster A.K.A Pryce. Forretress... 0 Atk Forretress Explosion vs. 0 HP / 0 Def Bellossom: 148-175 (98.6 - 116.6%) -- 93.8% chance to OHKO. Yeah no. Muk is an even worse matchup than Ariados, as it is much bulkier. Venomoth is a kinda neutral matchup because while +2 Attack Return has a high chance of OHKOing the moth, it can also Toxistall and Supersonic you until you die of Toxic damage. If you need to, hold a Lum Berry for this. Crobat is way too dangerous to fight because of Wing Attack's 3HKO compared to unboosted Return meager 4HKO. If you absolutely need to win this only using Bellossom, Sword Dance to 6+ when you battle Venomoth and hope Crobat doesn't use Double Team. (Suggested Level: 50)
Elite Four Bruno (Indigo Plateau, Fighting-type): Great. Hitmontop is not a threat, with Triple Kick achieving a 4HKO 72.9% of the time. Use it to set up 3 Sword Dances and 3 X Speeds (remember to heal in between) . Hitmonlee is also a joke, as Blaze Kick 3HKOs and it dies to anything Physical Bellossom throws at it. Hitmonchan is the same thing. Onix... is a Rock/Ground type. Machamp is fine as 6+ Attack Leaf Blade definitely OHKO it and at that Speed Bellossom outspeeds everything Bruno has. (Suggested Level: 50)
Elite Four Karen (Indigo Plateau, Dark-type): Not that good. Sword Dance up and hope Umbreon doesn't use Double Team for 2 turns if you are holding a Zoom Lens and 3 turn if anything else, then Drain Punch it to death. If it uses Confusion Ray even after 1 Sword Dance, switch. Staying in is as wise as stabbing yourself. Vileplume is not advised, but manageable as a whole. Any number of Sword Dance allows Bellossom to 1-2HKO Vileplume, while Vileplume's Acid, which is the only attack that it can use to damage Bellossom, 4-5HKOs. Murkrow is not scary, unlike Ariana's, as Pluck 3HKOs Bellossom and even +2 Attack OHKO back. Heal first if Bellossom have been doing the heavy lifting though. Gengar is a no - sure max Attack Leaf Blade OHKO, but Destiny Bond. That is enough reason for Bellossom to stay the hell away. Get something faster to deal with Gengar instead. Houndoom is a no-go: 0 SpA Houndoom Flamethrower vs. 0 HP / 0 SpD Bellossom: 102-120 (68 - 80%) -- guaranteed 2HKO. Critical Hits still exist, and unless you have +4 or higher Attack, Drain Punch will not cut it. (Suggested Level: 50)
Champion Lance (Indigo Plateau, Flying-type): Horrid. Gyarados is not a great matchup for Bellossom, as both party can 3HKO the other, but Gyarados is faster. It is ill-advised to fight any of the Dragonites, as their best moves - Dragon Rush, Fire Blast and Blizzard, the first of which they all have - 2HKOs the poor flower. Even max Attack Return can only 2HKO. Both Charizard and Aerodactyl can also 2-3HKO with their STAB, but this time you can OHKO them back at 4+ Attack with Return and Leaf Blade respectively. Still not a good situation to put your Bellossom in though. (Suggested Level: 50)
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.
Gym #9 - Brock (Pewter City, Rock-type): His entire team is OHKOed and has no means of hitting Bellossom for noticeable damage aside from Kabutops whose Rock Slide is a 3HKO. Go nuts. (Suggested Level: 55)
Rival (Mt. Moon, optional): As established before, Sneasel is a non-threat. Use it to get to +6 Attack, then murder it with Drain Punch to substitute for lost health. Golbat can be bad - it's Poison Fang 3HKOs. However, if you did the aforementioned thing, go ahead and Return it. Magneton goes down to Drain Punch. Keep your Bellossom at over half health is probably going to be enough for Alakazam - its Psychic 2HKOs and anything Physically damaging you do OHKO it. His Gengar is the same as Morty's - no. Avoid Typhlosion and off the other 2 starters. (Suggested Level: 55)
Gym #10 - Misty (Cerulean City, Water-type): Golduck's Psychic deals pathetic amount of damage, though it can be annoying with Disable. It's a good set up fodder for Bellossom to get to +6 Attack and, if you can, +2 Speed. Then pull a Bruno and let Bellossom sweep the hell out of Misty. (Suggested Level: 55)
Gym #11 - Lt. Surge (Vermilion City, Electric-type): Raichu is laughably weak, because of Quick Attack's... 9HKO chance. Yeah, use Sword Dance thrice. Then kill everything. (Suggested Level: 55)
Gym #12 - Erika (Celadon City, Grass-type): Her Pokemon can't do actual damage to Bellossom. Sword Dance up and sweep away. (Suggested Level: 55)
Gym #13 - Janine (Fuchsia City, Poison-type): All of her Pokemon can hit Bellossom Super Effectively. The users of this Pokemon should keep said Pokemon away from the battlefield. (Suggested Level: 55)
Gym #14 - Sabrina (Saffron City, Psychic-type): Espeon can boost itself up to sky-high and mutilates your Bellossom, while you need to get to +4 in order to secure a KO. Let somebody else handle this. Alakazam and Mr Mime can 3HKO you, so pick one. Should you want to use Bellossom on both, Sword Dance, X Speed, heal, X Speed again, heal again and get victory over those two and hope that Mr. Mime doesn't get a Critical Hit at all. (Suggested Level: 55)
Gym #15 - Blaine (Seafoam Islands, Fire-type): Are you insane? (Suggested Level: 60)
Gym #16 - Blue (Viridian City): No. Exeggutor is the only thing you can attack safely - its Psychic is a 3HKO. It knows Hypnosis though, so give Bellossom a Lum Berry. At max Attack, Return will OHKO, although you'll not use it after that, as everything else can kill it very quickly. Arcanine is Arcanine, Rhydon can pull a Pryce on you, Gyarados has Ice Fang, Machamp has Dynamic Punch, and Pidgeot has Air Slash. Stay away. (Suggested Level: 60)
Rival (Indigo Plateau, optional): Sneasel, Magneton, Alakazam, the starters and Gengar are exactly the same as before. Crobat's Air cutter has a ~50% chance to 3HKO, but Confuse Ray can be a big problem. Give Bellossom a Persim Berry and it'll be all well and good, due to +6 Return OHKOing back. (Suggested Level: 60)
Red (Mt. Silver): Pikachu's an easy one, due to Volt Tackle's 3HKO chance and Leaf Blade's chance of OHKOing back. Snorlax is workable, because even Giga Impact can only 2HKO. Sword Dance twice and use Drain Punch twice again. Blastoise might be alright if he sends it out next, with Leaf Blade OHKOing the former and the same is also true for Lapras. Charizard and Venusaur is a definite avoid though. (Suggested Level: 85)
At the time of capture, Oddish knows Absorb and Sweet Scent, which, let's be honest, are crappy moves. At just a few levels later, it gets Acid - it's first reasonable STAB move. Getting to Levels 13,15 and 17 nets you Sleep Powder, Stun Spore and Poisonpowder. All three are quite good for different purposes. And just when it evolves, it gets Mega Drain, which is pretty good, just quickly outclassed. As Gloom, it gets Lucky Chant at Level 29, Natural Gift at 35, Giga Drain at 47 and Petal Dance at 53. All horrid moves, but Petal Dance is kinda good, I guess. Moonlight, which comes at 41 is solid, though. As Bellossom, it's moveset is a lot less bad. Magical Leaf comes at 23, making Mega Drain obsolete. At 53, instead of Petal Dance, it gets Leaf Storm, which is much better than the alternative.
It's TM and tutor moveset is just as barren. On the Physical side, it gets Sword Dance, Drain Punch, Return, Facade for status and Leaf Blade from the Move Relearner. On the Special side, it gets Giga Drain, Grass Knot, Energy Ball and Hidden Power. Yeesh.
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.
What Nature do I want? Brave or Quiet is the best, as Bellossom uses both of its stats. Relaxed and Sassy can also work, as it don't use its Speed stat that much.
Which Ability do I want? It has only one.
At what point in the game should I be evolved? You should get a Bellossom right before Whitney, to ensure that Miltank is dead with a capital D.
How good is Bellossom in a Nuzlocke? Solid. It have its good and bad matchups, and while Johto in general isn't the best place for a pure Grass-type, Bellossom has the right coverage and bulk to get through Johto and go slaughter half of Kanto.