The 30-year-old, who was released by AFC Wimbledon earlier this month, signed two days after Constable made the switch to the Skrill South champions.
It brings the pair together for a second time, after they combined to good effect in the United squad which won promotion back to the Football League via the play-offs in 2010.
The Hampshire club are managed by former U’s midfielder Richard Hill, who felt a second big signing in succession showed they meant business next season.
He said: “Like James Constable, Jack is an extremely ambitious lad and, to come and play in the Conference, he needed to be convinced that we’re not going into this league just to tread water.”
Latest United news:
11:00am Friday 27th February 2015
GARY Waddock says he feels no bitterness towards Oxford United as he prepares to face the club for the first time since being sacked last summer.
9:20am Friday 27th February 2015
MICHAEL Appleton says there is no better place for Oxford United to win this season than Fratton Park – and it has nothing to do with his links to Portsmouth.
9:00am Friday 27th February 2015
THERE have been plenty of changes at Oxford United since they visited Fratton Park last season, but few players have undergone a more dramatic change of status in the 18-month gap than Callum O’Dowda.
8:10am Friday 27th February 2015
DANNY Hylton says he has found a system this season which is helping him take penalties with confidence.
8:00am Friday 27th February 2015
PAUL Robinson believes Oxford United could set out to frustrate tomorrow in response to Portsmouth’s recent results.