Points leader Martin Truex was going after his series-high fifth win of the season, first leading the pack off a restart with 10 laps remaining, but still needing the ideal restart a second time after Michael McDowell crashed, leading to a red flag to clean oil off the track with four laps left.
Larson won the Pure Michigan 400 Sunday at Michigan International Speedway for his third consecutive win at the Irish Hills track.
Chip Ganassi told Larson on the radio during the red-flag period before OT that he should go three-wide on the restart, but even Ganassi was surprised when the No. 42 split the No. 78 and No. 20. Truex finished second, with rookie Erik Jones third.
Elsewhere on Sunday, Dale Earnhardt Jr.is still searching for his first win in the 2017 NASCAR season, and Kasey Kahne would probably love to add at least one more to his resume before he exits Hendrick Motorsports at the end of the year.
Larson is the first driver to win three times in a row at MI since Bill Elliott did it from 1985-88. Keselowski, who earned pole position for the Pure Michigan 400, now sits in fourth place in the Monster Energy NASCAR Cup standings.
Truex Jr. leads Larson by 129 points in the point standings.