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 Vileplume when exposed to a Leaf Stone. Availability varies across all games; in Gold and Silver, the Leaf Stone is post-game only.
Completed analyses of Vileplume so far: