O'Neill was a member of the 2023 United States men's national lacrosse team at the offensive midfield position.

[2][3] A native of Bay Shore, New York, O'Neill began playing lacrosse in second grade.

He played varsity lacrosse in eighth grade for Bay Shore High School, recording 98 points to lead all of Long Island, before attending St. Anthony's High School in South Huntington.

O'Neill played three seasons of varsity lacrosse for the Friars before his senior season was canceled due to the COVID-19 pandemic, where he scored 153 goals and had 66 assists for 219 points, winning the CHSAA Long Island championship as a junior in large part due to O'Neill scoring seven goals against rival Chaminade in the championship game, and he was named the US Lacrosse Magazine National Player of the Year that season.

That season, he was the only player in Division I lacrosse to finish in the top 20 nationally in points per game, goals, and assists.