Jarrod King of Edinboro won it as a 12 seed back in 2009. Strangely enough, Jarrod had just transferred from OU previously.
Jake Rosholt of Oklahoma State won it as a 10 seed in 2003.
I remember back in 1994 Mark Branch for OSU won it with a losing record entering the tournament. He was obviously unseeded, and the last unseeded guy to win it all. But it was also his first of two NCAA Championships, and four trips to the NCAA Finals (1st, 2nd, 2nd due to injury in the finals against a kid he walloped earlier, and 1st).