It can be turned on by pressing the blue button inside the cabin, but not over the phone. So you have to wait for a locksmith if you lock yourself out; but at least no one is tracking unsuspecting vehicle owners who don’t turn on OnStar.
You may recall hearing that OnStar does track cars after deactivation. You may even have heard about that on a leading consumer news site. The company never actually tracked their ex-customers, but only proposed a change to their terms and conditions giving them permission to. A few months later, they reversed course due to the overwhelming negative reaction to that idea.