Undrafted, O'Neill played collegiate hockey with Yale University of the ECAC before signing a one-year entry-level contract with the Los Angeles Kings at the conclusion of his senior year on March 15, 2012.[1] and was assigned to the Manchester Monarchs.
In the 2014–15 season, his third full season within the Kings organization, O'Neill was assigned to continue to play with the Manchester Monarchs of the AHL. He enjoyed a breakout year offensively with the Monarchs, leading the team and the league with 58 assists and 80 points in 71 games to be awarded the Les Cunningham Award as the AHL's most valuable player. In the post-season, O'Neill's continued his offensive pace in recording 10 goals and 20 points to help the Monarchs dominate the league and capture the Calder Cup in their final season in the AHL.
Before the 2015–16 season, O'Neill attended the Kings training camp and participated in the pre-season. O'Neill was unable to earn a roster spot in Los Angeles, and as a result, on the eve of the season, on October 6, 2015, the Kings traded O'Neill to the New Jersey Devils in return for a conditional 7th-round pick.[2] O'Neill made his NHL debut with the Devils on October 10, 2015, against the Washington Capitals. He played 22 games in the NHL for the Devils that season and also made 42 appearances for their AHL affiliate Albany Devils.[3]
On May 20, 2016, he signed a deal with Helsinki-based club Jokerit of the Kontinental Hockey League (KHL).[4] In December 2020, O'Neill, dubbed "Mr. Helsinki", signed a new three-year contract with Jokerit.
O'Neill was an offensive catalyst for six years in Jokerit before the club withdrew from the KHL during the 2021–22 season due to the Russian invasion of Ukraine.
As a free agent, O'Neill opted to continue his career abroad, agreeing to a two-year contract with Swiss club EV Zug of the NL on May 6, 2022.[5]
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