Hey,
I am in the exact same boat as you. I became interested in OMS while considering med school. First of all, you need to consider general dentistry, Its what you will be learning in dental school and you will have a better experience if you enjoy it and find it interesting. I have a friend who is a D1 and although I was interested in med school, this guy showed me that dental school is by all means just as interesting.
Now back to OMS; keeping in mind what I just said, I will be taking the chance you mentioned, and gunning for this residency. I beieve that with hard work this is possible. As for competitivness, OMS is about the third hardest specialty to get into, behind Ortho and Endo. For OMS, several attributes while in school are 'required' 1) high class rank (Ive heard of as low as top 15% and as high as top 5%) 2)high board scores on NBDE1-around 90% 3)research experience and 4)a possible externship in OMS. A general trend that I have also heard as of late is that people are steering away from OMS due to the program length (4-6 yrs). For only 2 yrs in some of the other speacialties, you can make more $. I am not doing it for the $ however, but more a love for surgery. So keeping all of this in mind although matching OMS is difficult, it is by no means impossible. Just go into your first year KNOWING how hard you will have to work and this will be good motivation for you.
As for alternatives, 1)if you are unsucessful in matching OMS, you can 'scramble' and get into a first year OMS internship that is 1 year in duration. Although there is no garuntee here these internships will often feed you into a residency program. 2) millitary residencies for OMS exist that are less competitive. After this you will owe Uncle Sam some time however. 3)Periodontics is a highly surgical field that has a lot of overlap w/OMS. 4) a General practice resedency for 1yr or experience as a general dentist will often make you more competitive for a OMS residency. Hope this all helps, and good luck.