Nic Moore, SMU's leading scorer, went out with a bruised right knee with 17:12 remaining in regulation and the Mustangs leading 46-36.
The Gauchos quickly made an 18-3 run for their first lead 57-56 with 5:22 left. SMU retook the lead on Moreira's layup with 4:08 left and held the advantage until Gabe Vincent nailed a 3 with 10 seconds left to tie the score at 67 and force overtime.
SMU's Ben Emelogu hit a jump shot with 1:44 left in the extra period for a 75-69 lead. The Mustangs then made five free throws to seal the win.
Cannen Cunningham had 10 points and 10 rebounds for SMU (6-3).
Michael Bryson led the Gauchos (4-4) with 25.