Bradley took over as interim coach when Joe Paterno was fired in the aftermath of child sex abuse allegations against former defensive coordinator Jerry Sandusky.
On Friday, Bradley posted on his Twitter account that he finished packing up his office late Thursday night, and that he "walked out the doors proud with a lot of great memories and friends, and a better man."
Bradley was still part of the staff when O'Brien, the offensive coordinator of the New England Patriots, was hired Jan. 7 to replace Paterno. But it was widely expected that Bradley would not remain under O'Brien.
Bradley said he never interviewed to be part of O'Brien's staff and will pursue other options.
Former Auburn defensive coordinator Ted Roof was officially named defensive coordinator on Friday.
Bradley and Jay Paterno had interviewed to succeed Joe Paterno, who was at Penn State for 46 seasons and amassed a Division I-record 409 wins.
Jay Paterno, who served as quarterbacks coach under his father, announced earlier this week that he would not return to the staff for 2012.
Penn State on Thursday announced the hiring of six assistant coaches, including two holdovers from the previous staff. Defensive line coach Larry Johnson and linebackers coach Ron Vanderlinden will remain in their roles under O'Brien.
Stan Hixon, most recently the Buffalo Bills' receivers coach, has been named assistant head coach/wide receivers coach for Penn State. Mac McWhorter will coach Penn State's offensive linemen, Charles London will coach running backs and John Strollo will coach tight ends. O'Brien worked with McWhorter, London and Strollo during his time as an ACC assistant with Georgia Tech and Duke.
O'Brien is still finalizing the hires of defensive coordinator, secondary coach and quarterbacks coach. The Associated Press and other outlets have reported South Carolina assistant John Butler will join Penn State's staff as well.
O'Brien has said he will call offensive plays for at least the 2012 season.
"It was crucial to get an experienced, passionate and enthusiastic staff together quickly so they can hit the ground running," O'Brien said in a statement. "All of these coaches have varied and successful backgrounds coaching in the NFL, college and high school across the country. They have developed extensive relationships with coaches that will be vital in our recruiting efforts. They are excited to meet our current players, get on the road and become part of the Penn State football family."
Roof was named Central Florida's defensive coordinator Dec. 8 after leaving Auburn following the regular season. He previously spent three seasons as the Tigers' defensive coordinator, including the 2010 national championship season. Auburn was ninth in the country in rushing defense that year.
O'Brien also hired John Butler to coach the secondary on Friday. Butler was on staff at Auburn and South Carolina the past two seasons, respectively.
Roof was also defensive coordinator at Minnesota in 2008, and spent six seasons at Duke. He became the Blue Devils' head coach in 2003, going 6-45 before departing in 2007.
O'Brien was offensive coordinator at Duke from 2005-6. O'Brien and Roof were also on the same staff at Georgia Tech from 1995-2001 under George O'Leary, the current head coach at Central Florida.
Butler, who is from Philadelphia, decided to move back to his home state after a year with the Gamecocks. He told Gamecocks coach Steve Spurrier on Wednesday he was moving on.
With Joe Paterno's firing still a hot-button issue among alumni, the university Board of Trustees issued a statement Thursday, responding to questions raised at a "town hall" event in Pittsburgh with university president Rodney Erickson. Another such meeting was scheduled for Thursday night in King of Prussia, a Philadelphia suburb.
Paterno, the board said in the statement, was removed in November instead of being allowed to retire after the season because of "extraordinary circumstances" arising from the criminal allegations against Sandusky. The former defensive coordinator, facing multiple charges of sexually assaulting children, has maintained his innocence.
"The Board's unanimous judgment was that Coach Paterno could not be expected to continue to effectively perform his duties and that it was in the best interests of the University to make an immediate change in his status," said the statement from chairman Steve Garban and vice chairman John Surma.
"The details of his retirement are being worked out and will be made public when they are finalized," the statement continued. "Generally speaking, the University intends to honor the terms of his employment contract and is treating him financially as if he had retired at the end of the 2011 football season."
Information from ESPN reporter John Barr, ESPN.com Big Ten writer Adam Rittenberg and The Associated Press was used in this report.