Westbrook hit 10 of 16 field-goal attempts and Grant made 9 of 11 for the Thunder, who pulled away in the second quarter.
Paul George scored 17 points and Dennis Schroder added 16 off the bench as Oklahoma City, playing the second of road back-to-backs, won its fourth straight game.
Bradley Beal led Washington with a game-high 27 points.
Howard, who missed training camp, preseason and the team's first seven games due to a strained piriformis muscle in his lower back, scored 20 points on 7-of-8 shooting in 23 minutes.
John Wall had 19 points and nine assists but also committed seven turnovers as the Wizards lost their fifth straight game.
The Wizards turned the ball over 18 times, compared to 13 for the Thunder.
Oklahoma City's bench outscored Washington's reserves 48-32 and the Thunder shot 57 per cent from the field, including 15 of 32 from 3-point range.
The Thunder led by 29 at halftime and increased the margin to 85-52 early in the third. Washington started to chip away, and Wall's layup pulled Washington within 19 late in the third quarter but the lead was 110-85 to start the fourth.
The Thunder came in averaging 109 points per game.
Howard made his first six shots and the Wizards led by as many as 10 points in the first quarter before a 9-0 Thunder run gave them a 35-30 lead after one.
Oklahoma City poured it on in the second quarter. The Thunder outscored the Wizards 30-11 over the final 6:30 of the second quarter and led 79-50 at intermission.
The Thunder shot 62.5 per cent in the first half -- including 9 of 16 from 3-point range -- and scored 21 points off 15 Wizards turnovers.
Westbrook, Steven Adams and Grant each scored 15 in the half, while Howard had 19 for Washington.
(With inputs from agencies.)