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O'Connor made his Ballintubber club debut whilst still a teenager in 2010. A year later, he made his county debut for Mayo in the Connacht Senior Football Championship against London at Emerald GAA Grounds in South Ruislip after being called up by former Ballintubber manager James Horan who had given O'Connor his Ballintuber debut as well. O'Connor was named as the captain of Mayo in 2013 after having won two Young Player of the Year awards in the previous two years and led them to the 2013 All-Ireland Senior Football Championship Final. In 2014, he was awarded a GAA GPA All-Star Award after being the top scorer in the 2014 season and leading Mayo to the 2014 All-Ireland Senior Football Championship semi-finals.

In 2015, O'Connor was in the running for the Golden Boot and initially finished tied for top with Fermanagh's Seán Quigley. However, following a review of Mayo's championship winning DVD, the Gaelic Athletic Association realised they had incorrectly noted O'Connor's score as they had recorded him as having scored 1-6 but he had actually scored 1-7. As a result, O'Connor's tally was increased by 1 which allowed him to claim the Golden Boot.

In 2019, O'Connor became the highest scoring player in the All-Ireland Senior Football Championship, surpassing Kerry's Colm Cooper's record. He had done this by scoring in all of his previous 51 previous Championship matches prior to the record breaking match, except for one game against London in 2013 when he had been black carded.