At the end of last season, Williamson's injury-hit campaign with Wolverhampton Wanderers came to an end after the Championship outfit opted against extending his contract.
However, the experienced centre-back is unlikely to be short of options in the Football League and he has been given the opportunity to go through pre-season with Oxford.
The third-tier outfit have revealed that the 33-year-old - who lives close to the club - will work with the first-team squad while they are out in Portugal over the next week.
There is no suggestion that a deal could be in the offing, but the acquisition of a player who has also spent time with Newcastle United and Watford would represent a considerable coup for the club.
While Williamson made just six appearances for Wolves last season, he helped the team register four clean sheets in all competitions.