O'Connor had trials at Crystal Palace and Leicester City, then another with Leeds United in March 2017, where he impressed then-coach Garry Monk enough to secure a -year contract for an undisclosed fee from Limerick|. He initially joined the Under-23 team, but participated in the 2017–18 first team's preseason under new coach Thomas Christiansen, and was called up onto the bench for the EFL Cup match against Newport County on 22 August 2017. O'Connor was named the Leeds Under 23s squad's captain for the 2017/18 season. Due to an injury crisis in defence, on 3 April 2018, O'Connor was named by new manager Paul Heckingbottom in Leeds' first team for their travelling squad for Leeds EFL Championship game against Fulham at Craven Cottage although he was not named among the substitutes. His first team debut came as a central defender alongside Pontus Jansson as Leeds came back from 0–1 down to earn a home point against Sunderland, with O'Connor earning praise for his first senior game. On 16 April 2018, O'Connor was nominated as one of four players for Leeds United's Young Player of The Year award, which was won by Bailey Peacock-Farrell. On 29 June 2018, O'Connor signed a new two-year contract extension at Leeds, with the option of an additional year. After signing the contract, director of football Victor Orta said of O'Connor, "he will be a key player for Leeds United for many years to come," and that the club intended to send him out on loan for regular games.
On 3 July 2018, O'Connor joined EFL League One side Blackpool on loan until 31 May 2019, in order to gain first team experience. He made his debut on 4 August in Blackpool's 0–0 draw against Wycombe Wanderers. On 4 September, O'Connor scored his first goal for Blackpool, deep into injury time in a 3–3 draw at Moss Rose against Macclesfield Town in the EFL Trophy. Macclesfield emerged the victors after winning the resulting penalty shootout 5–3. O'Connor scored his second goal of the season, on 31 October, in a 2–1 defeat to Premier League side Arsenal in the Carabao Cup at the Emirates Stadium. O'Connor was also sent off late in the same game for a tackle on Arsenal striker Pierre-Emerick Aubameyang. His final game for Blackpool before being recalled early by Leeds came on 5 January 2019 when he started in a 0-3 FA Cup defeat against Premier League side Arsenal. In total he played 17 games for Blackpool scoring 2 goals in all competitions.
Bradford City
On 8 January 2019, O'Connor signed for Bradford City on loan until the end of the 2018–19 season after cutting his loan deal with Blackpool short. At Bradford, however, O'Connor has had limited game time due to Rule 54.4.1 of the EFL Rules and Regulations which states that, "A maximum of 5 players registered on a temporary loan transfer can be named in the players listed on a team sheet for any individual match played under the auspices of The League." However, after the appointment of Gary Bowyer as manager, O'Connor impressed in his short spell at Bradford City playing 9 games, with O'Connor becoming the Bradford City Captain for the remainder of the season under new manager Gary Bowyer. In June 2019 O'Connor returned to Bradford City on a three-year permanent contract. He scored his first goal for Bradford when he scored in an EFL Trophy tie against Bolton Wanderers on 3 September 2019.
