O'Mara won the Master'U BNP Paribas, the world event of university tennis, with Great-Britain in December 2017.

[citation needed] He made his Grand Slam debut at the 2017 Wimbledon Championships after receiving a wildcard to the doubles main draw with Clayton.

[1] In June 2018, O'Mara claimed his maiden ATP Tour title with Luke Bambridge in the men's doubles final at Valley International in Eastbourne.

[3] With compatriot Liam Broady he won the doubles at the 2023 Surbiton Trophy[4] and reached the final at the 2023 Nottingham Open.

[5] Partnering with Olivia Nicholls at the 2023 Wimbledon Championships, O'Mara reached the semi-finals in the mixed doubles, losing out to eventual champions Mate Pavić and Lyudmyla Kichenok.