Occupational therapy focuses on improving someone's functional ablilities. They often do not directly treat injuries, but help a person optimize their independence and ability to accomplish their daily activities and work, previous to an injury or physical ailment. Their goal is to improve a persons life skills and daily tasks.
Physical therapy focuses more on the injury, and diagnosing the physical source of the problem. They work with the person to treat and heal the injured tissues and structures.
Both OT and PT are very knowledgeable about musculoskeletal injuries and rehabilitaion. More so than a general practitioner.