Jakub Vrana, John Carlson and Marcus Johansson also scored for the Capitals, who have beaten the Sabres three times in 15 days, including twice during their current three-game winning streak - Washington is 1-2-1 against all other teams since beating Buffalo on Nov. 25.
''Those are important because they're almost like a little playoff series,'' Capitals coach Barry Trotz said of the recent games with the Sabres. ''Every game, because we're in the same conference, they're four-point games.''
Grubauer, the backup to Braden Holtby, is 4-1-1 with a 1.67 goals-against average and .940 save percentage this season. He has beaten Buffalo in his last two starts.
''I'm seeing them back-to-back almost,'' Grubauer said. ''Not much time in between, which is really helpful.''
''I don't think we did enough to win the game,'' Okposo said. ''We just didn't have that extra jam.''
Buffalo's penalty kill also struggled, allowing two goals. Sabres opponents have scored on the power play 19 times over the last 12 games.
''We have to find a way to get kills,'' Sabres defenseman Cody Franson said. ''You give teams like Washington chances on the power play, they did a good job for the most part, but the small chances we did give them, they were able to capitalize.''
Entering Friday, Washington was 3 for 28 on the power play in road games, ranking 26th in the NHL.
The Capitals improved to 14-3-2 when scoring first and 12-1-1 when leading after two periods.
Oshie gave the Capitals a 1-0 lead 6:43 into the second period when he lifted a shot from the right edge of the crease past Lehner. Jay Beagle drew two defenders into the corner and sent the puck back to a wide open Oshie for his ninth goal of the season.
''I feel like I'm back to normal,'' said Oshie, who returned Wednesday after missing seven games with a lower-body injury. ''My legs got underneath me and I feel like it's been a long time since I was out.''
Vrana scored his first NHL goal on a power play to make it 2-0 with 7:21 left in the second. Evgeny Kuznetsov set up the goal, skating in from the right boards and sliding a backhand to Vrana in front for the tap-in.
Grubauer made 10 of his saves in the second period, including one on a point-blank shot from Sabres center Jack Eichel that bounced off the back of the goaltender's skate.
The Sabres finally got a goal with 7:53 remaining when Okposo scored off a rebound on the power play.
Carlson put the Capitals back up by two goals when he scored his first of the season on a power play with 5:30 remaining. Carlson collected a long, rolling rebound at the point and sent a slap shot over Lehner's glove.
Johansson added his 12th goal of the year into an empty net with 1:12 left.
NOTES: Capitals C Nicklas Backstrom went to the locker room to be evaluated for an undisclosed injury late in the first period but was able to return for the start of the second. Backstrom leads Washington with 22 points. ... D Dmitry Kulikov, D Josh Gorges and F Nicolas Deslauriers returned to the Sabres lineup. Kulikov sat out 13 games with a lower-back injury, Gorges missed three games because of a non-displaced fracture in his foot and a knee injury kept Deslauriers out for 19 games. ... Capitals D Matt Niskanen did not make the trip after sustaining an upper-body injury in Wednesday's win over Boston. ... Sabres F Derek Grant missed the game due to an illness. ... Buffalo returned D Brendan Guhle to his junior team on Friday after the 19-year-old played in the past three games. ... Trotz coached in his 1,386th NHL game, tying Mike Keenan for ninth on the career list.
Capitals: Host the Canucks on Sunday night.
Sabres: Have three days off before hosting the Kings on Tuesday night.
Copyright 2016 by STATS LLC. Any commercial use or distribution without the express written consent of STATS LLC is strictly prohibited.