A transfer from Virginia Commonwealth, Okereafor hit 12 straight from the line in the final six minutes of regulation and scored a layup with one second left to force the first overtime.
Khallid Hart made two free throws with three seconds left in the first overtime to force the second.
Okereafor scored the first eight points of the second overtime to give Rider (5-12, 1-5 Metro Atlantic) a 100-96 advantage, then followed a Jimmie Taylor free throw with one of his own to create the final score.
Hart finished with 36 points to lead Marist (4-11, 1-5). Brian Parker added 28 points and grabbed 13 rebounds.