[2] He attended Seton Hall University and graduated from Florida Southern College,[3] where he played varsity baseball and basketball.

O'Brien came to the major leagues when he joined the Texas Rangers as a vice president during the 1973 season, and was promoted to general manager of the Rangers on September 2 of that year; he would continue in that post through 1978, although he shared power with co-general manager Eddie Robinson in both 1977 and 1978.

In 1979, O'Brien became president of the Seattle Mariners, then in their third year as an American League expansion team, and he added the general manager title to his duties during 1981–1983, replacing Lou Gorman.

He left Seattle in 1984 and joined the front office of the Cleveland Indians in 1986, working with Peters again as his top assistant during 1987–1989.

The Mariners climbed to the middle of the pack in the AL West in 1982, then fell into the basement with 102 losses in O'Brien's last year.