how do you evolve Onix? I tried metal coat but that didn't work.
You have to either:
Trade the onix to another trainer while it is holding the Metal Coat then get it back in a trade a second time.
Trade any Pokemon with Jasmine to get a Steelix with a soothe bell. You first have to defeat her in a rematch and trade with her between 1 and 2 PM in game.
Metal Coat isn't the only thing you need.
You need to TRADE an Onix holding a Metal Coat for it to evolve. Then trade back, to recieve your all new Steelix
If you're looking for Steelix you can get one at Mt. Silver and the Cliff Cave