That's a complicated question.
Pretty much everything outside of surgery and surgical sub-specialties are open for DOs in every region. This will not change in the future.
It's going to be harder and harder for DOs to match, especially after the merger takes full effect (mid 2020s) for surgery and subspecialty programs. The fact is that MSUCOM and OUHCOM did a lot of the heavy lifting in the past few decades, opening up a ton of surgery seats in their states and the rest of the COMs grads were taking advantage of them while opening little to no seats in their OPTIs. MSUCOM and OUHCOM have both expanded (more satellite sites, class expansion, etc.) and the state of Michigan has opened up a few new MD public schools. For example, I guess Beaumont Health, which had 2 DO surgery residencies affiliated with MSUCOM, will be a feeder program for OUWB, a new public MD school in Michigan.
DO schools are expanding and not opening up diverse GME opportunities in their OPTIs. And, the schools that did do their jobs, like MSUCOM, OUHCOM, NYCOM (now NYITCOM), WesternCOMP, etc, are being out-competed with new DO and MD schools opening. Another example, Broward Health near Miami, a hospital with many DO programs, was bought out by FIU because FIU promised to invest hundreds of thousands of dollars for GME expansion.
The fact is that matching surgery, weather MD or DO, is very tough. Getting into programs with in-house fellowships, like Baylor-Dallas or Cleveland Clinic, is even tougher.
I've started applying to electives and, believe me, most PDs, especially the younger ones, view DOs and MDs as equals. The only problem is that, in 80% of the cases, the MDs do indeed have a stronger application and we have to accept that. The other 20% of DOs that have as good apps as MDs, just apply in the DO match and save themselves the headache/gamble. This is for surgery. I can't comment on what will happen for IM/EM, etc.