Willem Brandwijk and Marquis Wright had 12 points apiece, Evan Hymes scored 11 points and Lavon Long chipped in 10 for Siena (7-10, 4-4 Metro Atlantic), which shot 50 percent from the floor.
Ogunyemi scored four points and Hymes had a layup as part of a 6-0 surge to give the Saints a 60-55 lead with 7:16 remaining. The Saints stretched the lead to 72-63 with 2:06 to play. Niagara's Karonn Davis scored five straight points to pull the Purple Eagles within 73-70 with 36 seconds left.
Wright split a pair of free throws, and Niagara had a turnover and missed a 3 to seal it for Siena.
Emile Blackman scored 21 points, Dominique Reid had 16 and Ramone Snowden 11 with five assists for Niagara (3-14, 2-6).