O'Reilly made his debut for NXT at the July 12, 2017, television tapings of NXT, which aired on August 2, losing to Aleister Black. At NXT TakeOver: Brooklyn III, O'Reilly, along with Bobby Fish, attacked Sanity after they won the NXT Tag Team Championship against The Authors of Pain. They then attacked newly crowned NXT Champion Drew McIntyre later that night, being joined by Adam Cole, turning heel in the process. The following month, the trio of O'Reilly, Cole and Fish was officially dubbed "The Undisputed Era".
On the September 20 episode of NXT, O'Reilly and Fish defeated Tyler Bate and Trent Seven due to interference by Adam Cole. On the October 4 episode of NXT, after Roderick Strong's match with Drew McIntyre, The Undisputed Era walked out and patted Strong on the back. On the October 25 episode of NXT, The Undisputed Era gave Roderick Strong an Undisputed Era armband, with Cole telling Strong that he is "not a loser" and offering him a position in their group. On the November 1 episode of NXT, The Undisputed Era attacked both SAnitY and The Authors of Pain during their match and it was announced that The Undisputed Era would be involved in the WarGames match at NXT TakeOver: WarGames.
On the November 8 episode of NXT, O'Reilly and Fish interrupted Roderick Strong vs. Adam Cole, leading to a massive brawl among the WarGames teams. At NXT TakeOver: WarGames, The Undisputed Era defeated SAnitY and the team of Roderick Strong and The Authors Of Pain to win the first War Games match in over 17 years. On the December 20 episode of NXT, O'Reilly and Fish defeated SAnitY to win the NXT Tag Team Championship. At NXT TakeOver: Philadelphia, The Undisputed Era defeated The Authors of Pain to retain the titles. At NXT TakeOver: New Orleans, The Undisputed Era defeated Roderick Strong and Pete Dunne and The Authors of Pain to retain the titles and win the Dusty Rhodes Tag Team Classic after Strong turned on his partner and joined Undisputed Era.
At NXT TakeOver: Chicago II, O'Reilly and Strong defeated Oney Lorcan and Danny Burch to retain the titles. On the June 19 tapings for the second annual United Kingdom Championship Tournament, O'Reilly and Strong lost the titles to Moustache Mountain (Tyler Bate and Trent Seven), but won them back two days later. At NXT TakeOver: Brooklyn 4, O'Reilly and Strong defeated Moustache Mountain to retain the titles. After the match, The War Raiders (Hanson and Rowe) attacked Strong and O’Reilly. On the November 14 episode of NXT, O'Reilly defeated Hanson, meaning Undisputed Era would have an advantage in numbers in the WarGames match at NXT TakeOver: WarGames. At the event, The Undisputed Era lost to Pete Dunne, Ricochet, and War Raiders (Hanson and Rowe). at NXT TakeOver: Phoenix, O'Reilly and Strong lost the titles to The War Raiders. On June 1 at NXT TakeOver: XXV, O'Reilly and Fish faced The Street Profits (Angelo Dawkins and Montez Ford), The Forgotten Sons (Wesley Blake and Steve Cutler), and Oney Lorcan and Danny Burch in a fatal-4 way ladder match for the vacant NXT Tag Team Championships, but the match was won by Dawkins and Ford. Fish and O'Reilly later challenged The Street Profits for the titles at NXT TakeOver: Toronto on August 10, but again failed to become the champions. Six days later on the August 16 tapings of NXT, however, O'Reilly and Fish finally defeated The Street Profits for the NXT Tag Team Championship. O'Reilly and Fish would go on to hold the NXT Tag Team Titles for several months until they lost to them to The BroserWeights (Pete Dunne and Matt Riddle) at NXT TakeOver: Portland. Following this, O'Reilly stopped appearing with Undisputed Era on television and only appeared in pre-taped segments. He made his in-ring return on the August 5, 2020 episode of NXT, where he and Fish unsuccessfully challenged Marcel Barthel and Fabian Aichner of Imperium for the NXT Tag Team Titles. On the September 16 episode of NXT, O'Reilly confronted Tommaso Ciampa backstage after his attack on Jake Atlas, thus turning face for the first time in his WWE career.
On the September 23 episode of NXT, O'Reilly defeated Kushida, Bronson Reed, Cameron Grimes and Timothy Thatcher in the first ever gauntlet eliminator match to challenge for his first singles title in the promotion, becoming number one contender for the NXT Championship at NXT TakeOver: 31 against Finn Balor. Following the match, Cole and Strong came to the ring to celebrate with him. At the event, O'Reilly was unsuccessful in winning the match. After NXT TakeOver: 31, O'Reilly and the rest of The Undisputed Era would begin to feud with the returning Pat McAfee and the NXT Tag Team Champions, Oney Lorcan and Danny Burch. At NXT Halloween Havoc, O'Reilly would confront the group while being accompanied by a returning Pete Dunne. This alliance would be short lived, as Dunne would attack O'Reilly with a steel chair and Pete Dunne would align himself with Team McAfee.
On the November 17 episode of NXT, The Undisputed Era, along with O'Reilly, would return and attack Team McAfee. NXT General Manager William Regal would announce that Undisputed Era would face Team McAfee in a WarGames match at NXT TakeOver: WarGames. At the event, Undisputed Era would defeat Team McAfee. On the December 16 episode of NXT, O'Reilly would face Dunne in a match where the winner would face Finn Balor for the NXT Championship at NXT New Years Evil. O'Reilly would defeat Dunne and earn himself another shot at the NXT Title. On January 6, 2021, at NXT New Year's Evil, he was unsuccessful once again. At NXT TakeOver: Vengeance Day, Bálor was attacked by Dunne, Burch and Lorcan after Bálor defeated Dunne to retain the NXT Championship; Undisputed Era aided Bálor to fend off Dunne, Lorcan and Burch. They then posed in the ring, where Cole attacked Bálor. When O’Reilly protested against Cole attacking Bálor, Cole then hit the superkick to O’Reilly, thus signaling the end of Undisputed Era. On the NXT after Vengeance Day, Cole once again attacked O’Reilly during the six-man tag team match between O’Reilly, Strong and Bálor against Dunne, Burch and Lorcan. Cole then hit O’Reilly with a brainbuster on the steel steps, causing O’Reilly to get stretchered out of ringside. O'Reilly returned on the March 11 episode of NXT and attacked Adam Cole after his title match against Finn Bálor. On the March 24 episode of NXT, O'Reilly and Cole would sign a contract to make their unsanctioned match at NXT TakeOver: Stand and Deliver official. On night two of NXT TakeOver: Stand and Deliver, O'Reilly would defeat Cole.
On the April 20 episode of NXT, he cut a promo on his goals of facing NXT Champion Karrion Kross when he was interrupted by Cameron Grimes, causing O’Reilly to sucker punch him and defeat him in a match later that night. O'Reilly then had a staredown with Kross and Scarlett. On the May 4 episode of NXT, he interrupted Kross once again. On the May 11 episode of NXT, Kyle defeated Oney Lorcan, and was attacked by Pete Dunne after the match until he was saved by the returning Bobby Fish. On the June 1 episode of NXT, O'Reilly faced Johnny Gargano and Pete Dunne in a triple threat match for an NXT Championship opportunity at NXT TakeOver: In Your House, which ended in a no contest after Cole returned and laid out all three men. Later that night, it was announced that Kross would defend his title in a fatal five-way against Cole, Dunne, Gargano, and O'Reilly. At the event, O'Reilly failed to capture the title after he passed out to Kross's Kross Jacket. After failing to win the championship, O'Reilly resumed his feud with Cole and a rematch between the two was made for The Great American Bash, where O'Reilly was defeated by Cole. He would continue to feud with Cole until he defeated Cole at NXT TakeOver: 36, ending the feud.
After his feud with Cole ended, O'Reilly would form an uneasy partnership with newcomer Von Wagner, who helped him fend off an attack from Pete Dunne and Ridge Holland. On the October 12 episode of NXT 2.0, O'Reilly and Wagner teamed up to defeat Dunne and Holland. Soon after, they declared their intentions for the NXT Tag Team Championship and on the November 30 episode, defeated Joaquin Wilde and Raul Mendoza to become the number one contenders. At NXT WarGames, O'Reilly and Wagner faced the reigning champions Imperium in a losing effort and after the match, Wagner attempted to attack O'Reilly, but O'Reilly would counter his attack, thus ending their partnership. On the following episode of NXT 2.0, O'Reilly faced Wagner in a steel cage match, but would be defeated and, after the match, would have his head smashed repeatedly with the steel cage door by Wagner. This would be O'Reilly's final appearance in NXT as his contract expired on December 10.