In July 2019, Askew joined the Portland test with Chip Ganassi Racing. On October 28, 2019, Arrow McLaren SP announced that Askew, alongside Patricio O'Ward, would race full-time for the team in 2020. A hard crash at the 2020 Indianapolis 500 left Askew with concussion-like symptoms; after racing the next four events, Askew sought treatment and was withdrawn from the harvest Grand Prix rounds. He was later cleared to return for the season finale. On October 12, AMSP dropped Askew in advance of 2021 without giving a reason why. Oliver Askew filled in for Felix Rosenqvist in the #7 AMSP entry at the second race of the 2021 Detroit Grand Prix, due to a crash the day earlier for Felix.