Khalil Alford's layup with 31 seconds left in extra time gave Rider a three-point lead. Okereafor stole it from Emmy Andujar - who fumbled driving to the hoop - with two seconds left, was fouled, and made a pair of free throws to seal it.
Rider used a 12-2 run late in the first half to take a 37-36 at halftime.
The teams traded the lead several times in the second half, and a layup by Manhattan's Ashton Pankey with 57 seconds left tied it at 74. Pankey was fouled, but missed the ensuing free throw, and both teams had misses in the last 32 seconds, leading to overtime.
Jimmie Taylor had 15 for Rider (12-7, 6-2 Metro Atlantic Athletic Conference) and Alford finished with 14.
Andujar and Shane Richards led Manhattan (8-9, 5-3) with 21 points apiece.