Wow, your numbers sound pretty good. I'm not sure about the 18/120 will go to OHSU, while the other 112 will go elsewhere, but everything seems to add up anyway. And, if it's any consolation, they'll accept more than 120 prior to May 15th; it's just that they don't exceed 120 filled seats. So, basically, the really considerate applicants who get accepted but choose to go elsewhere who withdraw from OHSU prior to May 15 will lead to more acceptances to fill the seats in the class.
Also, there are 120 people in my class, and the waitlist got to number 77, maybe? I don't know how many waitlisters took OHSU up on their offer, and how many direct admits did, but this means that between one-third and half the class came off the waitlist. (I was a waitlister, and I got in pretty soon after the rankings were released.)