O'Day opened the game with a layup and a shot block as Vermont cruised to a 16-4 lead. Trae Bell-Haynes added 10 points and six assists for the Catamounts (9-8, 4-0 America East).
Wayne Sparrow, who led the Retrievers (2-15, 0-4) with 20 points, poured in two layups and a jump shot to spark a 10-2 run that closed the gap to 18-16 and forced a Vermont timeout. Cody Joyce scored 14 and Jourdan Grant 12.
UMBC stayed close, and the first half ended 33-31, Vermont. The Catamounts opened the second half on an 11-2 run with three O'Day layups and a Kurt Steidl 3-pointer to break away.
Vermont made 25 of 46 field goals (54 percent) with 17 assists.