Who makes the decision as to when you return to work, occupational health or your doctor?

Both: Typically, the physician determines what you are capable of doing that is medically safe and won't exacerbate your injury (i.e. No lifting over 10 lbs, no repetitive bending or stooping, etc.) occupational health determines if there are job modifications available at your place of work that fit the limitations set by your physician.

Both: Your treating doctor will make his recommendation and then occupational health will translate that into how that fits to the work that you do. Ultimately the employer decides whether he is comfortable in letting the employee work.