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:
4:00pm Thursday 26th March 2015
OXFORD United have signed Peterbrough striker Kyle Vassell on loan until the end of the season.
8:00am Thursday 26th March 2015
Sam Long hit the opener and made the second as a youthful Oxford United development side beat their Oxford City counterparts 2-0 yesterday.
8:00am Tuesday 24th March 2015
Midfielder Michael Collins says head coach Michael Appleton’s calm persona will help banish Oxford United’s relegation fears.
9:30am Monday 23rd March 2015
MICHAEL Collins believes Oxford United’s players have to fight for their livelihoods and be willing to put their bodies on the line for the sake of the club’s Football League status, writes JACK JOHNSON.
8:00am Monday 23rd March 2015
Michael Appleton was left disappointed by Oxford United’s lack of cutting edge, after his side did everything but score against promotion-chasing Shrewsbury Town, writes JACK JOHNSON.