O'Brien was drafted by the Philadelphia 76ers in the first round of the 1976 NBA Draft. He quickly established himself as a key contributor to the team, averaging double-digit points and rebounds in each of his first three seasons. He was also named to the NBA All-Rookie Team in 1977.
In 1979, O'Brien was traded to the Portland Trail Blazers, where he continued to be a productive player. He was a key contributor to the team's run to the NBA Finals in 1980, averaging 12.2 points and 8.7 rebounds per game during the playoffs.
In 1983, O'Brien was traded to the San Antonio Spurs, where he played for one season before retiring. During his eight-year career, he averaged 11.2 points and 8.3 rebounds per game. He was also named to the NBA All-Defensive Second Team in 1981.
O'Brien's career was marked by consistency and reliability. He was a key contributor to the teams he played for, and his strong rebounding and defensive skills made him a valuable asset. He was also a leader in the locker room, and his presence was felt both on and off the court.
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