With the game tied 99-99 and time ticking down, Beal took the ball into the lane, shook off a pair of defenders and put up a soft shot from the hip that went in for the decisive score.
The Thunder's Russell Westbrook missed a shot at the buzzer, sealing a rare victory for the Wizards, who improved to 5-28 on the season and 4-13 at home.
Beal finished with 22 points, as did Martell Webster, while Kevin Seraphin added 19 as the Wizards beat the best team in the Western Conference.
Emeka Okafor provided a double-double of 12 points and 12 rebounds in the victory.
Kevin Durant netted 29 points -- including a game-tying 3-pointer 36 seconds left -- but it wasn't enough for the Thunder, who lost for the second time in four games.
Serge Ibaka added 26 points and 11 boards in Oklahoma City's loss.
Notable deaths of 2014 [PHOTOS]