Trailing by 14 in the first half, UC Davis (16-4, 6-1 Big West) battled back during the second and tied the game at 65 with Ritchart's 3 to send the game into overtime.
In OT, Cal Poly (10-10, 3-5) twice took the lead with layups from David Nwaba before Avery Johnson sank the go-ahead 3-pointer to give the Aggies the lead for good 72-71. Josh Fox and Corey Hawkins helped pad the advantage and Johnson sealed it with a pair of free throws with 3 seconds to go after Cal Poly again closed to within a point.
Hawkins finished with 22 points and Johnson 11 as the Aggies shot 50.9 percent from the field.
Brian Bennett led the Mustangs with 27 points.