Can my occupational health doctor send me back to work?

Yes. Yes they can, unless you are under workman's comp and that doctor is not your assigned doctor.
Yes. If you are being treated by another doctor you occ doc will most likely get his opinion first. Then the occ doc will make a recommendation to your boss. The employer makes the final decision.
YES. If your injury was a result of a workers comp accident your occupational doctor can send you back to work.