Oscar started karting in 2011, taking multiple Australian championships when he became professional in 2014. He started to compete in European and other CIK-FIA sanctioned championships driving for Ricky Flynn Motorsport.
For the 2016 season, he moved to Formula 4 UAE Championship driving for Dragon F4. Piastri finished 6th in the standings, with a standalone car for the team.
He started his F4 British Championship debut with TRS Arden Junior Team, competing in 30 races. Oscar won 6 races in Oulton Park, Snetterton Motor Racing Circuit, Knockhill Racing Circuit and Silverstone Circuit and had 13 podiums. With 376.5 points, he finished 2nd in the championship behind Jamie Caroline.
Oscar made his Formula Renault debut with Arden driving in the Northern European Cup and the Eurocup. In the NEC championship, he was only a guest driver meaning he was ineligible to score points in the championship. In the Eurocup championship, he competed as a rookie and finished 8th in the standings, taking 3 podiums in Spa-Francorchamps and Hockenheimring. For the following season, he once again raced in the Eurocup category but this time with R-ace GP, driving alongside Caio Collet, Alexander Smolyar and Grégoire Saucy. Piastri won 7 races and got 11 podiums across the 20 races (19 that he competed in). With 320 points, he finished 1st in the standings ahead of Victor Martins with 7.5 points.
FIA Formula 3 Championship
Oscar Piastri was announced to be driving his rookie season in Formula 3 with Prema Racing, the previous champion dominating the championship with Robert Shwartzman, Marcus Armstrong and Jehan Daruvala. He will be driving alongside Frederik Vesti, the previous Formula Regional Champion and Logan Sargeant, who previously drove for Carlin in the Formula 3 championship.