Just for conversation, builder I think you risk very little by using the the firing pin without heat treatment.
David is correct that the proper way to harden and temper O-1 requires specific times and temperatures. You can look it up and see that it would be impossible to do by feel or sight. On the other hand, if you do want to try the hardening, O-1 will definitely get harder using basic approaches. No doubt, my comment about tempering in an oven will leave the pin harder than necessary, if it did in fact achieve maximum hardness, but again it was a thought to simplify your home approach. It'll work out, have fun with it.
