Early career O'Malley trains in
Glendale, Arizona, at The MMA Lab led by head trainer John Crouch, and is coached by his long-time friend, Tim Welch. O'Malley was given the nickname "Sugar" by one of his MMA coaches in Montana early on in his career. After his win over Nuzzo, O'Malley earned a spot on
Dana White's Contender Series 2 where he took on Alfred Khashakyan. O'Malley beat Khashakyan with a stunning knockout in the first round and was awarded a UFC contract.
Ultimate Fighting Championship O'Malley's promotional debut came against
Terrion Ware on December 1, 2017, at
The Ultimate Fighter 26 Finale. O'Malley won the fight via unanimous decision. O'Malley next faced
Andre Soukhamthath on March 3, 2018, at
UFC 222. He won the fight by unanimous decision, despite injuring his foot in the third round. This fight earned both participants a
Fight of the Night bonus. O'Malley was expected to square off against
José Alberto Quiñónez on October 6, 2018, at
UFC 229. However, O'Malley announced that he was out of the fight on September 30 due to a potential Anti-Doping Policy violation. On October 25, O'Malley received hip surgery while waiting to receive judgment pending his potential violation. Subsequently, O'Malley was suspended for six months by the
NSAC after testing positive for
ostarine. He became eligible to return as of March 2019. O'Malley was scheduled to face
Marlon Vera on July 6, 2019, at
UFC 239. However, O'Malley announced his withdrawal from the bout on June 21, 2019, due to a failed test for
ostarine. The
Nevada State Athletic Commission decided to suspend him due to the failed test and he was suspended for 6 months by
USADA. The
ostarine in his system was likely residual from his previously failed test prior to
UFC 229. O'Malley fought
José Alberto Quiñónez at
UFC 248 on March 7, 2020. He won the fight via TKO in the first round. This win earned him his first
Performance of the Night award. O'Malley faced former
WEC Bantamweight Champion Eddie Wineland at
UFC 250 on June 6, 2020. He won the fight via one-punch knockout in the first round. This win earned him the Performance of the Night. O'Malley laced up against
Marlon Vera in the co-main event of
UFC 252 on August 15, 2020. He would go on to lose the fight via TKO in round one, the first defeat of O'Malley's career. He was later diagnosed with
drop foot, which was the result of a leg kick landed by Vera. O'Malley faced
Thomas Almeida on March 27, 2021, at
UFC 260. During the first round O'Malley dropped Almeida with a punch, and walked away thinking that Almeida had been knocked unconscious. Despite this he won the fight via knockout in the third round. This win earned him the Performance of the Night award. O'Malley was scheduled to face
Louis Smolka on July 10, 2021, at
UFC 264. However, Smolka pulled out of the fight in late June citing injury, and he was replaced by promotional newcomer Kris Moutinho. He won the bout via TKO in round three, after outstriking Moutinho with hard shots for all three rounds. This fight earned him the Fight of the Night award. O'Malley faced
Raulian Paiva on December 11, 2021, at
UFC 269. He won the fight via TKO in round one. The win also earned O'Malley his fourth
Performance of the Night bonus award. O'Malley faced
Pedro Munhoz on July 2, 2022, at
UFC 276. Early in the second round, O'Malley accidentally poked Munhoz in the eye, rendering him unable to continue. The fight was declared a no contest. O'Malley faced former
UFC Bantamweight champion Petr Yan on October 22, 2022, at
UFC 280. He won the bout via split decision. The result was seen as highly controversial, with many fans and fighters expressing their belief that Yan was the rightful winner. 25 out of 26 media outlets scored the fight for Yan. As a result, the editors of
Sherdog website gave this fight "2022 Robbery of the Year Award". The bout received the
Fight of the Night bonus.
UFC Bantamweight Champion O'Malley faced
Aljamain Sterling for the
UFC Bantamweight Championship on August 19, 2023 at
UFC 292. He won the title by second-round technical knockout at the beginning of the round. This earned him another
Performance of the Night bonus. He defended his title via unanimous decision. This earned him his sixth
Performance of the Night bonus award. For his second title defense, O'Malley faced
Merab Dvalishvili on September 14, 2024 in the main event at
UFC 306. He lost the fight by unanimous decision. On September 16, O'Malley revealed on his podcast that he suffered from a torn labrum from before the fight and that he will undergo surgery. O'Malley faced
Merab Dvalishvili in a rematch for the championship on June 7, 2025 at
UFC 316. He lost the fight via a north-south choke submission at the end of the third round.
Return to contention O'Malley faced
Song Yadong on January 24, 2026 at
UFC 324. He won the bout by unanimous decision. 12 out of 21 media outlets scored the bout for O'Malley. O'Malley is scheduled to face
Aiemann Zahabi on June 14, 2026 at
UFC Freedom 250. == Submission grappling ==