I let DS touch the oven if it isn't hot. That way, when it is hot, and I say & sign 'hot', he knows not to touch. I did let him touch it once when it was getting hot, and he pulled his hand away bloody quickly. That reinforced the 'hot' message. You have to be careful with this though - a friend of mine tried it with her DS and the oven was too hot, and his reflexes were fast enough to pull away before he was burnt.
The knobs are a work in progress though. I say no very very firmly and send him out of the kitchen. If he doesn't go, I bodily remove him to the living room. He doesn't like that, so he's fiddling with them less often. I think the message is getting through.
I think NOELallie's idea is the best for the long-term. When did you start? DS is nearly 18 months and although he's very interested in watching me cook, I'm not sure he has the skills yet to join in.