O'Callaghan made his football championship debut for Dublin against Laois in the quarter-final of the 2016 All-Ireland Senior Football Championship. He scored a single point against Laois as Dublin progressed to the Leinster semi-final.[2] He made his second appearance against in Dublin's semi final victory over Meath and continued this trend in the final against Westmeath.[3][4] He failed to make an appearance outside the Leinster Championship but won a medal as a squad member of the 2016 All-Ireland Champions Dublin.
2017 started with O'Callaghans first start as a senior inter-county footballer with Dublin. He started the game against Carlow in the quarter final of the Leinster Championship. He scored two points from frees and failed to score from play in the victory over a very defensive Carlow side.[5] In the Leinster semi final against Westmeath, he scored his first points of the 2017 campaign from play. He scored a total of three points from play in Dublin's victory over Westmeath in the semi-final.[6] In July 2017, he won his second Leinster senior football championship with Dublin what proved to be his most prolific game from a scoring point of view. He scored a total of 0–12 points against Kildare at Croke Park and helped guide Dublin to their seventh consecutive Leinster Championship title.[7] He took six frees during the game and scored six points.[1]
By 2019, his physical development had reached remarkable proportions. Never physically light to begin with, he sported a shaven head to add to his tougher image.[1] According to Alan Brogan, O'Callaghan had built a gym at the back of his house.[1] In the 11th minute of a game against Roscommon, O'Callaghan collected a pass by Brian Fenton from above his head past the "D" and fell onto his twisted right leg.[1] His former Dublin under-21 manager Dessie Farrell later remarked on the fall: "It was a model for a cruciate [injury], wasn't it?... I thought he was hurt the way he landed there for sure, but it's just a mark of his athleticism".[1] O'Callaghan rose from the awkward fall without effort, immediately hopped the ball and sent it over the bar for a point.[1] In the All-Ireland semi-final, he scored two goals past Lee Keegan and took a fierce hit from Mayo goalkeeper Rob Hennelly but was unaffected.[1]