taus sort of answered the question, but let me clarify it a little bit more for you...
DOs do NOT necessarily have to do an extra year in comparison to MDs. Certain specialties (as mentioned above) require a traditional rotating year. I would say most do, actually. ie. you want to do an ENT residency, you have to do a tradidional rotating surgical internship year. In the recent years, many residencies have been building this intern year into the actual residency curriculum. this goes for both MDs and DOs.
5 states require that DOs do an AOA approved internship. PA, OH, MI, FL, and one other, i can't remember which. this counts as that traditional rotating internship year i mentioned in the previous paragraph, it just counts as AOA approved intern year because it has a certain amount of primary care requirements built into it that you have to meet. however, only a limited number of places have them (mostly in the states that require them, with others scattered about.)
regardless of whether or not you choose to do an AOA year, 90% of the time you're going to do some kind of intern year anyway. you would be wise to choose an AOA one if you plan on practicing in the aforementioned states. also, i think you have to have completed one if you ever plan on being a program director of an osteopathic residency (not that you would probably know that now ...)
i do believe the only time you might end up doing some extra work is if you did a traditional rotating intern year, and then chose a residency that had the intern requirements built into it ... but that could happen to anyone who didnt do their research, MD or DO ...
some please correct me if im wrong or clarify what i've written, the whole intern year thing is a little bit confusing
to the OP, i took both the DO and MD boards. for me, it was more for curiosities sake, and because i didnt want to regret not taking them, on the off chance it could help me be a more competitive residency applicant. you certainly dont need them. and by law, im pretty sure no allopathic program can require them (legally, that is, but thats probably not realistic)