You'll need to give it a Leaf Stone to evolve it into Vileplume or use a Sun Stone to evolve it into Bellossum.
Chat with our AI personalities
my oddish evolved at level 21 maybe around the 20s
In SoulSilver, Oddish will evolve into Gloom at Level 21 however Gloom can evolve 2 different ways, you can use a Leaf Stone on it in order for it to evolve into Vileplume or you can use a Sun Stone on it in order for it to evolve into Bellossom.
Personally i would say as soon as you get gloom evolve it.
Your Aron will evolve at level 32 into Lairon.
Oddish evolves into Gloom at level 21.