He signed with the Green Bay Packers in 1979 as a defensive end and also played in the Canadian Football League for the Toronto Argonauts in 1980.

In 1983, he moved to the University of Nevada, Las Vegas, where he served as UNLV's linebackers and special teams coach from 1983 through 1985.

O'Brien then joined the Denver Broncos as their special teams coach in 2007, leaving after the 2008 season to serve the New England Patriots in the same capacity, re-joining Belichick.

On February 3, 2015, two days after O'Brien's special teams unit helped the New England Patriots win Super Bowl XLIX, the Patriots announced that O'Brien would retire from coaching after 24 seasons in the NFL.

[3] O'Brien was a member of the staff when the Patriots defeated the Atlanta Falcons in Super Bowl LI.