I've checked Serebii.net and it says that it's found in Granite Cave, but other sites say it's not in Granite Cave. I'm having a really difficult time finding one, please help me!!
Onix can be found in Granite Cave using the DexNav. Onix has a 50% chance of appearing, and can be found in areas 1, 2 and 3 of Granite Cave.
Also Onix will be level 12 in all three areas.
I would tell you how to use it, but I'm not sure myself, because I don't have ORAS yet.