The fact of the matter is, while you think you may want to specialize in whatever, rather than go into primary care, you may change your mind. I worked in the ER for a long time back home before school, I'm president of the EM interest group, and I'm doing emergency medicine research this summer at one of the nation's busiest level 1 trauma centers. So you could say that I'm interested in EM. But I'm not going to pretend that I know enough about medicine in other areas to say that I know for sure that i'll end up in EM.
Just like you with OMM. You don't have to know a lot about it to say that it interests you. Even if you've seen it done, or had it done to you, you won't understand any of the physiology behind it unless you've studied kinesiology or athletic training or something similar. But if you say you're interested in cardiac physiology, does that mean you have to know how to do an echo?
You're pre-meds. You're not expected to know ****. And if you sit down at your interview with some OMM guru who has been doing it for 30 years, and you start talking out your ass about OMM (or any other aspect of medicine), you're going to look like a ******. But that doesn't mean you can't be interested. Just get a basic knowledge of what it is and why its done. Come to your interview with questions about it. This is not a big deal.
"I work with DO's in the ED, but have never seen it done. However, I've read a little and talked about it to them a little, and it seems like it has a legitimate place in certain aspects of medicine, and it interests me" is a viable thing to say, in my opinion.