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. Sean was given the nickname "Sugar" by one of his MMA coaches in Montana early on in his career. When Sean asked why this nickname was chosen, his coach responded, "Because you're sweet to watch". O'Malley fought his first five fights of his career in his native Montana, before leaving to fight in North Dakota and eventually fought for Legacy Fighting Alliance where he got a highlight reel knockout over David Nuzzo. After his win over Nuzzo, O'Malley earned a spot on Dana White's Tuesday Night Contender Series when he took on Alfred Khashakyan. O'Malley beat Khashakyan with a stunning knockout in the first round and was awarded a UFC contract.
O'Malley's promotional debut came against Terrion Ware on December 1, 2017 at. 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 face 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 faced 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 ChampionEddie 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 is scheduled to face Marlon Vera on August 15, 2020 at UFC 252.
O'Malley competed at Quintet: Ultra as a member of Team UFC on December 12, 2019. He submitted Takanori Gomi with a guillotine choke in his opening round of the tournament but was subsequently submitted by Hector Lombard with an ankle-lock. The team advanced to the next round and faced Team Strikeforce. In that round, he went to a draw with former Strikeforce championGilbert Melendez, being both eliminated of the round. Team UFC ended up winning the tournament.
Personal life
O'Malley is a co-host, with friend and head coach, Tim Welch, on their own podcast, The Timbo Sugarshow. O'Malley has been an endorsed athlete of prominent combat sports brand, Sanabul, since 2017. He was previously following a plant-based diet but has since gone back to incorporating meat into what he eats. O'Malley is an advocate for legalizing marijuana.
