Think about it like this... Osteopathic medicine teaches medicine from the framework of a philosophy (some guy developed some guidelines on how effective medicine of all types should be practiced and personally I like these guidelines). Part of this philosophy emphasizes the importance of the musculoskeletal system and manifests itself in our medical education as additional courses which are basically a combination of physiology and orthopedics.
Allopathic medicine teaches medicine in the absence of any real articulated philosophy (which is not to say that it is lesser in any way).
In the end, however, both MDs and DOs receive the same core medical education and have the same unlimited licenses to practice medicine of all types. Most importantly, both are held to the same legal "standard of care." So if you present to the ER with crushing chest pain you aren't going to receive different care from the two different professionals.
Does this clear it up for you?
(Future DOs, please note that I explained the difference between the two branches without using the words MANIPULATION or HOLISTIC *and* without speaking negatively of the other branch.)