O’Regan spent parts of his childhood in Germany, where he started skating. After his father had retired from professional ice hockey, the family returned to the United States, settling in Needham, Massachusetts. He attended Roxbury Latin School in West Roxbury, Massachusetts, before transferring to St. Sebastian’s School in Needham, Massachusetts, after his sophomore year. He attended the USA Hockey's National Team Development Program and subsequently enrolled at Boston University in 2012, leading the Terriers in points as a freshman, while making the Hockey East All-Rookie Team. He received Hockey East Second Team honors as a junior, helping BU win the Hockey East championship and was named to the All-Regional Team. An assistant captain his senior year, he was named a Hockey East First Team All-Star. O’Regan played 154 games wearing a Terriers' uniform, tallying 66 goals and 88 assists to become the first BU player to pass the 150-point mark since 1998. In April 2016, he signed a two-year entry-level contract with the San Jose Sharks of the National Hockey League. He was initially assigned to AHL affiliate, the San Jose Barracuda to begin the 2016–17 season. On November 21, 2016, O'Regan was recalled from the Barracuda by the San Jose Sharks to make his NHL debut against the New Jersey Devils. He was scoreless as the Sharks were victorious in a 4-0 shutout effort. O'Regan was soon returned to the Barracuda and continued to lead the club in scoring in his first professional season. In 63 regular season games, O'Regan compiled a league best 58 points as a rookie, helping the Barracuda to their first pacific division title. He added 7 points in 15 post-season games before suffering defeat in the Western Conference finals to the Grand Rapids Griffins. O'Regan was awarded as the AHL's Rookie of the Year and earned a selection to the AHL All-Rookie Team. During the 2017–18 season, on February 26, 2018, the Sharks traded O'Regan along with a conditional first-round and fourth-round pick in the 2019 NHL Entry Draft to the Buffalo Sabres in exchange for Evander Kane. He was called up by Buffalo for the first time on March 7, 2018, reuniting O'Regan with his two linemates from Boston University, Jack Eichel and Evan Rodrigues. On July 1, 2019, O'Regan was signed as a free agent to a one-year, two-way contract with the New York Rangers.
