The answer is "it depends":
If you plan to go into a program that requires a separate PGY-1 first -- Derm, Rad, Neuro, etc -- then it's a definite possibility. The key here is that it is completely dependent on what the appropriate board feels. Last I checked, the Amer Board of Derm had no rules at all about what happens in the PGY-1 -- one of my Derm prelims missed a bunch of time due to a medical illness, and in the end didn;t have to make up any of that time. The Amer Board of Psych and Neuro, on the other hand, has strict rules about what the neuro people have to do in their internships -- I have had a few PGY-5/6's in our neuro program (after finishing a fellowship), come back for a few months of PGY-1 work to complete their training because their original PGY-1 year didn't meet the board's criteria.
If you want to transfer into a PGY-2 medicine slot, then if the Osteo internship is dual accredited (ACGME and AOA) then you will get credit -- if it is AOA only then you will get little or no credit -- maybe three months, probably zero. A dual accredited program will also have a PD who can advocate for a PGY-2 spot for you in other ACGME programs. PGY-2 spots are not listed on any central list (like the match) -- you simply have to call and find one. It can be difficult, there are threads on this topic for which you can search.