O'Shea made his debut for the Mayo seniors against New York in 2009 and since then has been one of Mayo's best players, winning an All-Star in 2013 and playing in midfield in two All-Ireland football finals, the 2012 decider, which Mayo lost by 0–13 to 2–11 against Donegal and the 2013 decider, which Mayo lost by 1–14 to 2–12 against Dublin.[1][2][3] In 2013, his man-of-the-match display drove Mayo to a 16-point victory in a rematch against 2012 conquerors Donegal at the All-Ireland quarter-final stage.[4] He was afterwards refused entry at one of Dublin's biggest nightclubs.[5][6] He was awarded the GAA's Player of the Month for August 2013.[7]
He played in the first Test for Ireland against Australia in the 2013 International Rules Series, but club commitments ruled him out of the second Test.[8][9] He is suspected of being concussed up to seven times.[10] In 2016 Mayo were denied another All-Ireland title by just one point to Dublin after a replay on 1 October. Coincidentally the following year Mayo lost again against Dublin in the 2017 All-Ireland Senior Football Championship Final by another one-point margin. O'Shea has lost in a lot of All-Ireland finals along with Lee Keegan in 2012, 2013, 2016, 2017, 2020, and 2021.[11]