Gallen made his inter-county debut in the 2019 Dr McKenna Cup.[2] Not part of the Donegal panel at the start of the 2019 season, he was called up over the course of the McKenna Cup as Donegal were short on numbers.[12] He made his league debut in the third fixture of the 2019 competition against Tipperary, starting and scoring three points (including one free).[13] He again started the fourth fixture and scored a point against Fermanagh.[14] He started the fifth fixture against Armagh.[15] He started and scored another point in the sixth fixture against Cork.[16] He started the seventh fixture against Kildare, scoring four points.[17] Donegal qualified for the National Football League Division 2 final and Gallen started the game and scored four points as Donegal defeated Meath to win the title.[18]
Gallen made his championship debut against Tyrone in the 2019 Ulster Senior Football Championship. He made another substitute appearance against Cavan in the final and scored a point.[19][20] He later picked up an Ulster medal, his first at any level of play.[21]
In October 2019 (during the Gaelic football off-season), Gallen attended an AFL Draft Combine in Melbourne, Australia. He later explained: "To get away for two weeks to Australia to try out in a professional sport was something I couldn't turn down. It was curiosity more than anything. You want to see what the level there is like and you want to see is there anything you can pick up or learn… I was up against the top young athletes in Australia and there hopefully are things there, standards I can take back home and apply to Gaelic football".[22]
When the 2020 National League resumed in October, Gallen started and played 45 minutes of the league game against Tyrone, before being substituted for Andrew McClean.[25] Gallen then played 63 minutes of the game away to Kerry, before being substituted for Conor O'Donnell, on what was the Carndonagh footballer's debut.[26][27][28] Gallen then made a substitute appearance against Tyrone in the 2020 Ulster Senior Football Championship quarter-final, scoring one point.[29] He made another substitute appearance against Armagh in the semi-final, again scoring a point.[30] However, Gallen sustained an injury during training ahead of the final and could not play.[31]
Gallen made a substitute appearance in Donegal's opening fixture of the 2021 National Football League, against Tyrone.[32] He started the next game, against Monaghan, scoring three points.[33] Gallen did not feature in the third game against Armagh, though he started the last game against Dublin.[34][35] He made a substitute appearance against Derry in the 2021 Ulster Senior Football Championship quarter-final.[36] He was named on the bench against Tyrone in the semi-final, but not play in the game.[1][37]
With his playing time often limited by injury in the early years of his career, Gallen started a senior inter-county championship game for the first time against Clare in the 2023 All-Ireland Senior Football Championship.[2] His nine-point tally against Derry's 2022 All Star recipient Chrissy McKaigue in Donegal's next game brought Gallen to national attention.[2] He then scored five points against Monaghan and four points against Tyrone.[2]
Gallen won an All Star at the end of the 2024 season.[40] He was nominated for one in 2023.[41][42]
^ abMcNulty, Chris (25 July 2021). "Oisin Gallen's return inspires MacCumhaills to derby win in Convoy". Retrieved 25 July 2021. Gallen, an unused substitute last Sunday as Donegal's Ulster Championship hopes fell asunder against Tyrone, started like a man on a mission. The 21-year-old landed a pair of early frees…
^"McHugh chasing elusive first medal". Donegal News. 21 June 2019. Retrieved 21 June 2019. Jason McGee… has an Ulster Minor Championship medal, so that leaves [Eoin] McHugh and newbies Oisin Gallen and Aaron Doherty as the only members of the squad who haven't claimed provincial honours with the county.
^Bonner, Declan (29 October 2020). "Always an edge when Donegal face Tyrone". Donegal News. p. 63. Young Conor O'Donnell made his debut [in the NFL away fixture to Kerry] and wasn't daunted by it.
^Bonner, Declan (26 November 2020). "Painful defeat in Ulster final". Donegal News. p. 63. Oisin Gallen and Ciaran Thompson picked up injuries at training last Tuesday night. Oisin wasn't able to play, while Ciaran was only able to come on as a sub…