Okay... hot-button topic here... I'll try to keep it short because I should be studying for my next licensure exam... T-minus 5 days and counting...
Basically, Osteopathy as a discipline was started in the 1870's in Kirksville, Missouri by Andrew Taylor Still,
M.D.. We won't get into the whole history here. A quick googling will bring up tons of sites on Osteopathy's history. Good place to start is here:
http://history.aoa-net.org/
Now, as far as what it was intended to be and what it has evolved into, that's where things get tricky...
It is safe to say that, in most instances, the "woo-woo" stuff of osteopathy has faded into the past. Most students in the U.S. who go to osteopathy school do so now because they could not secure a spot in the more competitive allopathic programs (i.e., M.D. granting). That's not a slam at the D.O. degree at all, and one should not infer that just because a D.O. gets a osteopathy degree doesn't mean he or she doesn't know his or her stuff.
In fact, the osteopaths are certified, by their education, to sit for the exact same licensure exams (the USMLE) as allopaths. Many of them take these (in addition to their own licensure exam called the COMLEX) and go into allopathic residencies alongside of M.D.'s They complete the same exact post-medical school training in many instances (and, this is the most important part of a training physician's education - this is where you learn your craft, supervised, and truly become a "doctor").
So, D.O.'s are, in essence, equivalently trained as M.D.'s, with a few EXTREMELY minor differences in the curriculum. However, one of the bigger differences is that they have to learn OMM - Osteopathic Manipulative Medicine. This is where opinions start to diverge...
Now, most osteopaths, if not the vast majority, never intend to utilize OMM. They don't believe in it, they don't practice it, but it's part of the undergar med curriculum. However, there are practitioners out their who are militant, and I mean militant, about OMM and it's benefits. They use opportunities at lectures to slam allopathic medicine, and offer chiropractic-like glorifications of what are, in my mind, questionable practices.
The studies these OMM zealots due are prone to all of the same problems that chiropractic studies are (e.g., difficult to blind, Hawthorne effect, hard to construct sham controls, etc.), but the way you'd hear some of these guys talk you'd think they could have cured polio with OMM. I've witnessed it firsthand. And, it was all I could do one time to sit through a lecture and not say anything, especially when this one D.O. (older gentleman) basically said that the triptans were a waste of time and money for migraines - and that he personally cured dozens of migraineurs with OMM. FEH!
So, I would tell you this, in general: Most D.O.'s don't do OMM and find little practical value in it. There are D.O. surgeons now, D.O. anesthesiologists, etc. where OMM is just... well, it would be silly to attempt to incorporate it into a, for example, laproscopic cholecystectomy. However, there still exists a vocal minority who, in my opinion, are no better than chiropractors and they use any opportunity to publicly take cheap shots at allopaths. Now, a lot of this stems from a long history of allopaths rolling their eyes at them, but you get the drift.
Osteopathy and the D.O. degree is becoming more and more prevalent, the younger generation of doctors coming out now are basically not OMM zealots, and D.O.s can do pretty much everything a normal allopath can do. So, there's no reason not to see a D.O. and have utmost confidence that they will treat you fairly... unless they start telling you to come see them twice a week and to throw away your heart medicine!
If I have more time later, I'll talk about the why the AMA is turning a blind eye to the sudden explosion of D.O. schools in the U.S. and the huge influx of International Medical Graduates (including a record number entering residency this year), both truly foreign and Caribbean-trained, in the U.S. along with the impending physician shortage by the year 2020, reversing COGME's long-held opinion that there was a 'surplus' of doctors in the U.S.
This one goes very deep and it has huge tentacles. But, I've got to get to bed so I can get up early and keep cracking the books...
Wish me luck!
-TT