Seriously? My gosh...do a search. This topic is covered at least 5 times, weekly.
But, to answer you question: DO's have their own residencies that are only open to them at this point. And yes, DO's can do MD residencies, albeit, the competitive specialties are quite hard to get into as a DO. In other words, most DOs going for derm, plastics, ortho, opth, etc. will end up doing AOA residencies, with exceptions here and there.
As for difficulty...it is hard to get into certain residencies as either an MD or DO. Some suggest that DOs have an easier time with competitive spots because of the AOA match, but I don't think there's any evidence that supports that as of yet. If you do well on your boards, rotations and have solid reports, you should be in good shape in either case.
In the future, search for these questions you may have. There is an FAQ that answers most all questions, and even the wikipedia page comparing MD and DO does a great job at answering basic inquiries.