Treecko in Pokémon Omega Ruby is not found in the wild; you normally get it as your starter from Professor Birch at the beginning of the game.

Where it comes from

  • Starter choice: Treecko can be selected as one of the three starter Pokémon from Professor Birch.
  • Route 101 mention: Some guides list Route 101 in connection with Treecko, but the reliable guide here still says the Pokémon is not wild-catchable in Omega Ruby.

Simple answer

If you want Treecko in Omega Ruby, the standard way is to choose it at the start or trade for it later.

Extra note

If you were asking about where Treecko appears in other games , some sources list Route 101 for different titles or contexts, which can cause confusion.