After his father had retired from professional ice hockey, the family returned to the United States, settling in Needham, Massachusetts.

O’Regan played 154 games wearing a Terriers' uniform, tallying 66 goals and 88 assists[4] to become the first BU player to pass the 150-point mark since 1998.

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.

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.

[8] 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.

[9] 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.

[15] At the end of his tenure with MoDo, O'Regan continued his career abroad in agreeing to a one-year contract with Chinese based Kunlun Red Star of the KHL on July 28, 2024.