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:
2:00pm Friday 27th February 2015
While Gary Waddock’s time in charge at Oxford United was brief, it produced one clear benefit which has helped the side this season.
1:00pm Friday 27th February 2015
NO further contract talks are expected at Oxford United until the club’s Football League status is secured.
12:00pm Friday 27th February 2015
Oxford United travel to Portsmouth today - catch up on all our preview stories and video of the first league meeting between the sides
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.