walk any where you should find it
Chat with our AI personalities
Onix is quite rare, but you can find him/her in the Oreburgh Mine or Mt. Coronet (but if you're willing to take the time to train it, you should go to Oreburgh Mine you'll find one pretty quickly)
Oreburgh Mine - Bulbapedia - Bulbagarden
go into oreburgh mine and talk to gym leader
You can find an Onix in Dark Cave or in Victory Road in the main game.
Find the crystal onix, make it weak and catch it with your best ball