How to remove a bad paint job on retro oven

A. Use a heat gun ⇦ on eBay or Amazon [affil link] on some high temperature paint?....;^)

Just kidding. You're probably looking at either a chemical stripper and some elbow grease or mechanical means such as sanding, grinding, etc. If it's old, be careful whatever you do as the old paint may have lead in it. I would suggest the proper mask and protective gear if you mechanically remove the paint.

If it's not too big, I would probably try some chemical stripper first. You can buy a small can and test it on a spot to see how well it's going to work. Then you can buy more if it does. If it's real big or has lots of recesses/nooks/hard-to-reach-spots-for-a-stripper, you might want to try finding someone who does sandblasting and see how much it would cost to go that route.