I have an osteoma on my forehead; is it better to go to a plastic surgeon or an orthopaedic surgeon to get it removed?

You are best off seeing a plastic surgeon.   I have done many of these removals using a very simple approach.   Under local anesthesia a small incision is made behind the hairline. Through this incision a small file is used to file down the bump. It takes about 30 minutes and costs between $2000 and $2500.   many perform it endoscopically which is quite a bit more involved and quite a bit more expensive.

You should go to someone who does a lot. In NYC, facial plastics with ENT training does this surgery frequently. I perform this with an endoscope with tiny ports in the hair to avoid scars. Choose your surgeon wisely.