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.