In June 2016, it was reported that Dain had signed with WWE to perform on their NXT brand. On 4 November, Dain appeared at an NXT live event and cut a promo. He made his in-ring debut at an NXT live event on 11 November, defeating Rich Swann under the ring name Damian O'Connor. He then reverted to using the ring name Damo.
On the 7 December episode of NXT, Damo attacked No Way Jose and seemed to align himself with the heel stable SAnitY. In January 2017, he began to wrestle under the new ring name Killian Dain. On the 18 January episode of NXT, Dain joined SAnitY in an official capacity after attacking Tye Dillinger and accepting Sawyer Fulton's jacket from Eric Young. On the 25 January episode of NXT, Dain appeared as part of SAnitY for the first time, attacking Chris Atkins by Young's order. On the 8 February episode of NXT, Dain teamed with Young and Alexander Wolfe to defeat Dillinger, No Way Jose and Roderick Strong. Earlier in the night, Dain and Wolfe defeated The Bollywood Boyz. At WrestleMania 33, Dain made it to the final three in the André the Giant Memorial Battle Royal before being eliminated by eventual winner Mojo Rawley. On the 19 July episode of NXT, Dain lost to Drew McIntyre in an NXT Championship number one contender's match, marking his first televised loss.
On the 28 March 2018 episode of NXT, Dain was named as one of six participants in ladder match at NXT TakeOver: New Orleans to determine the inaugural NXT North American Champion. He, however was unsuccessful in winning. On the 18 April episode of NXT, Dain lost a No Disqualification match to Lars Sullivan.
During the 2018 WWE Superstar Shake-up, SAnitY (excluding Nikki Cross) were drafted to SmackDown. The group debuted on the 19 June episode of SmackDown Live, where they attacked The Usos. The following week, SAnitY lost to The Usos and Jeff Hardy. On the 3 July episode of SmackDown Live, the group attacked The New Day. This led to a tables match between the two teams at Extreme Rules, which SAnitY won. On the 24 July episode of SmackDown Live, SAnitY and The New Day faced off in the first round of the WWE SmackDown Tag Team Championship number one contender's tournament. SAnitY (represented by Dain and Wolfe) were defeated.
The group competed in only two more televised matches throughout the remainder of 2018, while primarily competing at live events.
After a near four month hiatus from television, SAnitY returned on the April 2, 2019 episode of SmackDown Live, losing a three-on-one Falls Count Anywhere match to The Miz.
During the 2019 WWE Superstar Shake-up, Young was drafted to Raw. With Wolfe joining NXT UK, the group was officially disbanded.
Dain made his return to NXT live events on 16 May, defeating Jermaine Haley. On the July 3 episode of NXT, a promo advertising Dain's return was aired. He made his return on the July 17 episode of NXT, attacking Matt Riddle following Riddle's match against Arturo Ruas. This led to a match between the two being scheduled for the August 7 episode of NXT. However, Dain attacked Riddle before the match could begin. On the September 25 episode of NXT, Riddle defeated Dain in a number one contenders street fight match for the NXT Championship, ending the feud. Shortly after, Dain would enter in a three-way feud with Pete Dunne and Damian Priest. At NXT TakeOver: WarGames, Dain faced Dunne and Priest in a triple threat match for number one contendership for the NXT Championship, which Dunne won.
In September 2020, he formed a dysfunctional tag team with Drake Maverick, turning face in the process. On June 25, 2021, Dain was released from his WWE contract.