Someone who practices OMM is a licensed physician (DO, for the most part). Someone who practices physical therapy is a physical therapist, not a physician. Thats all they can do is physical therapy. OMM is just one tool that a DO can use, and his/her scope of practice is not limited at all. As a DO you can be a surgeon, pediatrician, family practice, ER, or whtever it may be, and you can use OMM in your practice if it is applicable.
Physical therapist go to PT school. When they graduate, their only option is some aspect of physical therapy.
DOs go to medical school, learn OMM, graduate as fully licensed physicains ready to practice any field of medicine, and are free to use OMM if they choose.
EDIT: Or was your question what is the difference between OMM techniques that DOs use, and the techniques and things that physical therapists use?