Onix does not evolve by level, it evolves into Steelix by trade whilst holding a metal coat.
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Onix doesn't evolve at a level. You have to give it the item 'Steel Coat' and trade it to make it evolve into Steelix. Or just catch a steelix at Victory Road using dusk balls
Onix doesn't evolve through leveling. But, make him hold a Steel Coat, and he will evolve into a Steelix!
Steelix
Onix will evolve by doing this: Have it hold a metal coat trade it to a friend then take it back you should receive Steelix.
Onix doesn't evolve from anything. It evolves into Steelix, but Onix is the base pokemon for that evolution chain. :)