I'm surprised not too many people have replied to your questions. Perhaps because the answers can be found relatively easily (or by asking your clinical instructors at your school). Anyway, here's my take on the situation from my research, and current students please correct me if I'm wrong:
- Osteopathic residencies take place either in osteopathic hospitals or in allopathic ones that have allowed osteopathic programs. Osteo hospitals are not very numerous, and they're usually smaller than allo hospitals, so generally the residency programs there are of relatively lesser "prestige" and sometimes you see a smaller number of cases compared to allo hospitals. Many allo hospitals, however, have "dually-accredited" programs (AOA and ACGME). An example for you would be Maimonides, which is an allopathic hospital, but NYCOM students can rotate there and also do their osteo internship too.
- DO students can apply to both osteo and allo residency programs, but MD students can only apply to allo programs. However, if you apply to both, the DO match occurs before the MD match, and if you match into a DO program you will be automatically dropped from the MD match (yes, the NBOME will notify the NRMP of your match and the NRMP will drop you). So, if you're thinking of applying to DO programs as a back-up in the event you don't get into your MD residency choice, it won't work.
- A combined internship...Hmmm...not absolutely certain of that one, but it sounds to me like a "linked" internship. An internship that will lead you automatically to the PGY2 year. Sometimes you apply just to the internship and before you complete it you then apply again to a residency program, or sometimes you can apply to an internship that is "linked" to the residency program (sometimes called "fast-track" programs). For example, if you know you want to do internal medicine, you can apply to an internship that emphasizes IM and that will automatically lead you to your first residency year in IM. Not all residency programs have a fast-track option. Current DO residents can probably give you more (and better) info on this.
- A traditional internship is an internship that isn't linked to a program. You rotate through all the hospital's departments, pretty much like in your clinical years, and gain experience in different aspects of medicine (ob/gyn, surgery, etc.). After your trad internship, you then apply to a residency program. A DO residency program will not take you without a trad internship, and the vast majority of MD programs will not grant you credit for the year you spent rotating as a DO trad intern. Many DO students complete a DO trad internship then apply to an allo residency program. The reason is that many DO students want to do allo residency programs instead of DO ones, and there are some states that will not grant you a license to practice as a DO unless you've completed the internship year (Florida, Michigan, Oklahoma, Pennsylvania, and West Virginia).
- A transitional year, as I understand it, is an allopathic internship either for people that still don't know what they want to go into, or for people that need a broad base of knowledge before going into their specialty. MD students can probably explain a transitional year better. Haven't looked into it much myself.
Sooo....are you confused enough? The AOA has a nice FAQ web page that explains the whole messy post-graduate situation for DO's. Check it out at:
http://www.aoa.net/PostDoc/ogmefaq.htm