In 2012, O'Connell lost in the first round of qualifying for the Caloundra Challenger while attending the Australian Institute of Sport on a tennis scholarship.

In January 2017, O'Connell made his ATP World Tour debut after qualifying for the Sydney International in his hometown.

O'Connell then received a wildcard for the 2017 Australian Open[12] where he lost his first round match in straight sets to (then) world number 15 Grigor Dimitrov in his Grand Slam debut.

The 2018 season was interrupted by a knee injury for O'Connell [14] where he was only able to play 24 matches across the Futures, Challenger and ATP World Tours finishing the year with a singles ranking of 1185.

After the restructuring of the ITF/ATP points system and the launch of the ITF World Tour, O'Connell returned to the Futures circuit to start his 2019 season.

[15] "I had a fair bit of time off, so after coming back it takes longer to get into the swing of things," O'Connell, said.

5 and leading the Tour in Finals made with 10, O'Connell returned to the ATP Challenger Tour in July recording wins in San Benedetto, Sopot and Tampere, most notably defeating former world number 5 Tommy Robredo in straight sets in their second round clash at the 2019 BNP Paribas Sopot Open.

Adjusting back to the Challenger Tour quickly, O'Connell made a breakthrough at the 2019 Internazionali di Tennis del Friuli Venezia Giulia in Cordenons, Italy, claiming his first Challenger Tour title with a straight sets victory over German Jeremy Jahn in the final.

[17] In September, O'Connell reached the final of Sibiu Challenger, which saw his ranking inside the top 200 for the first time.

"[19] O'Connell then followed his title with a series of strong results on the Challenger Tour, making semi-finals in Las Vegas and Houston & a final in Knoxville where he was defeated by Michael Mmoh in straight sets.

O'Connell was awarded a wildcard into the 2020 Australian Open, where he lost in four sets to 17th seed Andrey Rublev.

[7] In February, O'Connell reached the semifinals of the 2020 Challenger Banque Nationale de Drummondville, before the COVID-19 shutdown.

Upon recommencement, O'Connell won his first Grand Slam match, defeating Laslo Djere at the 2020 US Open.

In his first participation in his career in the main draw at the French Open as a wildcard, O'Connell lost in the first round to Tommy Paul in a tight five sets match.

[7] In July, O'Connell qualified for and won his fourth and fifth career ATP main draw matches at the Atlanta Open.

[23] In Atlanta, O'Connell reached his first ATP quarterfinal and registered his first win over a top 30 player, defeating Jannik Sinner in the second round.

He also upset Albert Ramos Viñolas after qualifying for Geneva before losing to 4th seed Reilly Opelka in the second round.

In July, at the 2023 Wimbledon Championships he reached the third round of Grand Slam for a second time but lost to Chris Eubanks.

At the 2024 Miami Open he reached the fourth round of a Masters for the first time in his career, having never been past the second round at this level, defeating qualifier Vit Kopriva, 21st seed Francis Tiafoe and wildcard Martin Damm Jr..[36] He lost to eventual champion Jannik Sinner.

[37] At the 2024 US Open, he reached his third Grand Slam third round with wins over 26th seed Nicolás Jarry and Mattia Bellucci.