Like faf said, a DO stands for Doctor of Osteopathy.
Bottom line where Osteopathy differs from allopathy is the coursework in OMT. Over 200 hours are spent in osteopathic schools teaching Osteopathic Manipulative Therapy. This therapy is not to be confused with chiropractics. All other aspects of training are the same. The same books are used, the same classes are taken. Osteopaths have residencies in ANY specialty(don't listen to this rural family hogwash).
At the end of the day, I want to be the kind of doctor who gives my patients the best care possible. If they have a problem I will use the most effective, least invasive means to help them. The New England Journal of Medicine has published papers about the efficacy of osteopathic techniques for things like low back pain. It works. Doing anything less would not be serving my patients to the best of my ability. Harvard and other medical schools are starting to add courses that include these techniques.
As a DO I will hopefully have the ability to treat a patient using more than my scalpel and prescription pad. Will this help every patient? No. Will I use this with every patient? No. Will I deny a patient surgery if they need it? No. Will I hesitate to give an antibiotic or pain medication if it is needed? No.
You have an infection, you take an antibiotic.
You have an inflamed appendix, you get it surgically removed.
You have a migraine...ahh...here is where I can maybe try something else along with or even before I reach for the medication. Again, will it work for everyone? Of course not.
Medicine is a very personal thing. Each patient respond differently to medicine, surgery, other treatments, and to the attitude of the physician. I, as a physician, will need to find the best way to take care of my patients. This can include different treatments or even referring them to a more qualified person. But I feel that as DO I may have more options.