Oddish is available in all Johto, Hoenn, and Kalos games, as well as more than half of the Kanto games. It can be caught in Sinnoh, but only in the post-game. The only region where it is fully unavailable is Unova. It evolves into Gloom starting at level 21, and from there, it can evolve into either Vileplume when exposed to a Leaf Stone, or into Bellossom when exposed to a Sun Stone. Availability varies across all games as far as the evolutionary stones are concerned.
You can find more information on the Oddish line in the branch-specific pages: