Oddish will evolve at Level 21.
You can find an oddish in the safari zone next to lilycove town
First get an Oddish train it to level 25 then use a leaf stone on Gloom.
level 19
36
at level 30
It eveloves at level 42
Oddish evolves at level 21 + it evolves into vileplume once you give it a Leaf stone.
level it up to level 33
Drowzee evelove at level 26
if you trade it then you get dugdrion i think
at lvl.41