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:
6:12pm Friday 29th August 2014
OXFORD United have loaned out Aidan Hawtin and Josh Shama to Banbury United plus Matt Bevans to Farnborough.
12:00pm Friday 29th August 2014
NEGOTIATIONS will continue over the next few days as Oxford United attempt to sign one more attacking player before the transfer window closes.
9:20am Friday 29th August 2014
MICHAEL Appleton is happy to sacrifice style in favour of substance tomorrow if it means Oxford United finally get up and running in Sky Bet League Two.
9:00am Friday 29th August 2014
AFTER spending 18 months away from competitive football, Oxford United’s newest signing may be short on match fitness, but there is one commodity he does not lack – self-belief.
8:40am Friday 29th August 2014
JOSH Ruffels is hoping Oxford United get a change of fortune as they attempt to get off the mark tomorrow.