O'Brien's move from the coaching staff of the New England Patriots to Penn State had been widely reported Thursday.
"We have found a leader who will embrace (Penn State's) legacy and maintain the university's commitment to excellence on the field and in the classroom," Erickson said.
Patriots owner Robert Kraft said O'Brien would remain on the job as New England's offensive coordinator during the playoffs. New England has a bye past the opening round of the postseason and will be in action on Jan. 14.
O'Brien, 42, takes over a program shaken by a child sex abuse scandal that has led to criminal indictments of school personnel and the dismissal of Joe Paterno, whose 409 victories are the most for a coach in the history of college football. Both O'Brien and Paterno attended Brown University.
"We have found the man to take Penn State football forward," said acting Athletic Director Dave Joyner. "Needless to say, we have been looking for someone with some very special qualities, beginning with a heart that beats to the values and vision of Penn State University and our Penn State football legacy and tradition.
"That was our starting point, and coach O'Brien exemplifies those traits that Penn Staters hold so highly. In addition to his model characteristics as a man and a teacher, he's all about producing winners, and doing so the right way. He will embrace tradition, demand excellence and pursue Success with Honor in every phase of our program."
"I cannot tell you how excited I am to get started, meet the team, meet the football alumni and meet all of the people that make this university so special," O'Brien said in a statement released by the school.
"As head coach of this special football program, it is my responsibility to ensure that this program represents the highest level of character, respect and integrity in everything we do. That includes my coaching staff, our players and everyone involved in the football program."