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:
1:57pm Saturday 20th December 2014
All the news from Oxford United's Sky Bet League Two clash at Hartlepool United.
9:30am Saturday 20th December 2014
Andy Whing says Oxford United should expect to beat Hartlepool United this afternoon despite the Football League’s bottom club appointing a new manager this week.
8:30am Saturday 20th December 2014
Michael Appleton’s main selection decision for Oxford United today is picking who leads the line against Hartlepool.
2:00pm Friday 19th December 2014
BRIAN Howard’s situation at Oxford United is unclear after head coach Michael Appleton admitted he did not know whether the midfielder would be offered an extension to his short-term contract.
12:00pm Friday 19th December 2014
JOSH Ashby is good enough to play in Oxford United’s side right now, but the teenager knows his time will come, according to head coach Michael Appleton.