it's definately very difficult. or at least it has been thus far as a third year d.o. student.
the fact is that many places don't want d.o.'s in their program. i even had a hospital tell me i couldn't even do an elective there as a 4th year b/c i'm not a m.d. stu and it's considered one of weakest programs around.
at the end of the day i think it comes down to how badly you want it. hard to know when you're just starting med school, i'd have to conceed. but if you get really great scores, impress the right ppl on showcase rotations, you have a shot (no guarantee) of mathcing- it won't be at wills/columbia type place, but a shot a some place (prob community hospital type setting).
another issue is at most d.o. schools (including mine) there's no advisor for you nor an ophthalmology dept. so you have to spend ALL your spare time just finding out info. no one to make calls for you or give adivse. you have to go though 6 degrees of separation to find someone who knows someone at an ophth residency somewhere or doc affilicated with a program in some way just to get basic info that others are fed but you are not privy to b/c at a DO school it's all on you. especially big ones like mine.
all that being said, there are a handful of DOs that match every year. "you gotta have faith", the one not annoything thing fred durst ever said.
pm if you have any other questions. although as a 3rd yr i'm limited. sorry.
best of luck!
