O'Brien put Tony Stockman and Velimir Radinovic on the bench and inserted Ricardo Billings and Ivan Harris into the lineup.
Stockman and Billings responded to the change in a big way as they helped lead the Buckeyes to their first win of the year, a 77-71 decision in a consolation bracket game at the Maui Invitational.
OSU (1-2) advances to meet the winner of today's game between Villanova and Santa Clara in Wednesday's fifth-place game at 2 p.m. Eastern time.
Stockman came off the bench and joined Terence Dials in leading OSU with 14 points. Dials also grabbed 14 rebounds. Billings added a career-high 13 points in his first career start.
Matt Sylvester and J.J. Sullinger each had eight points. Radinovic added seven and Brandon Fuss-Cheatham scored six.
Gerrit Brigitha led CMU (0-2) with 13 points, while Brandon Voorhees and Kevin Nelson each had 10 points.
OSU led 42-38 at halftime and moved ahead 68-58 after Stockman fed Sullinger for a layup and Dials for a slam.
CMU trimmed the gap to 71-68 with a three-pointer with 1:42 left. Sullinger then missed a pair of free throws, the second one an airball, with 1:27 left. But Fuss-Cheatham stole the ball back with 1:00 left.
Billings made four free throws and Sullinger made two in the final 18.3 seconds to put the game away.