Manager Michael Appleton was keen to seal a quick deal for loanee striker Kemar Roofe after the 22-year-old ended the season with six goals in seven games for the U's.
An undisclosed fee has been paid to West Bromwich Albion and Roofe has signed a three-year contract. He is joined by Liam Sercombe after the 25-year-old midfielder opted to leave Exeter City and join their League Two rivals.
New Portsmouth manager Paul Cook has allowed nine players to leave Fratton Park, including former Northampton loanee Danny East while Stevenage are set to hold talks with Teddy Sheringham over replacing Graham Westley if he decides to leave the club upon the expiry of his contract shortly.