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:
5:00pm Monday 2nd March 2015
OXFORD United have strengthened their attacking options before tomorrow's clash with Morecambe by signing Armand Gnanduillet on loan.
10:55am Monday 2nd March 2015
WATCH the key moments as Oxford United stood firm to claim a goalless draw at Fratton Park.
9:30am Monday 2nd March 2015
BOSS Michael Appleton hailed a new-found trust with his Oxford United players as they delivered a disciplined performance to claim a point with a goalless draw at Portsmouth.
9:00am Monday 2nd March 2015
RYAN Clarke was thrilled to preserve his perfect record against Matt Tubbs on the way to a hard-fought clean sheet at Fratton Park.
8:40am Monday 2nd March 2015
JAKE Wright was delighted to shut out what he felt was one of the best attacks in Sky Bet League Two as Oxford United drew 0-0 at Portsmouth.