This is one of the times I actually agree with MeatTornado. I remember posting about this concern a while ago and this was pretty much what I was thinking. There are still unfilled AOA spots every year in the match. Now with the merger, these spots can be accessed by US-IMGs and FMGs. Thus the percentage matching will go up initially for them. However, it is difficult to say how much. There will be those AOA programs that cannot meet ACGME standards and thus have to shut down (this is after the ACGME tries every effort to keep them open). This will be bad for DOs because now there will be less AOA spots to potentially apply to (I am strictly speaking from a number standpoint). It is difficult to say how things will change in the long run, because with increased competition and residency spots being capped, there will be those that may not match. Hopefully, DOs will be favored in the long run and increasingly so.
I think the real benefits comes from a few things that DrEnderW has pointed out. 1) DO students don't have to fear dropping out of the ACGME match and can apply to where they want (as long as expectations are within the realm of reason) 2) DO students that did AOA residencies can apply to fellowships and won't be barred out 3) When applying for a job after residency, with the ACGME label behind their training, and hospitals will have an idea of the quality level (because in the past AOA residencies are extremely variable, some really good and some really bad).