The Aggies (13-15, 4-12) had erased much of a 12-point first-half deficit, trailing by three entering the locker room, but the Cowboys (14-15, 7-9) scored nine of the first 10 points out of halftime to take a 38-27 lead and force an A&M timeout.
Back-to-back buckets by Ray Turner would pull the Aggies within seven, but a Keiton Page jumper followed by a dunk and three-pointer by Williams regained the momentum for good for OSU.
Page added 15 for the Cowboys, while Markel Brown poured in 12.
Oklahoma State took a 3-point lead into the locker room thanks to an 18-2 run that spanned six minutes of the first half. With the Cowboys leading 26-15, A&M began a strong comeback-mixing in two threes by Elston Turner and one each from Dash Harris and Khris Middleton-to cut the Cowboy lead to one. A bucket by Cezar Guerrero with 1:57 left proved to be the half's final points.
Middleton led the Aggies with 14 points while David Loubeau added 10.
Texas A&M returns to action on Tuesday at 6 p.m. when the Aggies host Kansas State at Reed Arena on Senior Night. All Aggie students get in free to the game.