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:
5:56pm Saturday 18th October 2014
4:57pm Saturday 18th October 2014
OXFORD United climbed out of the bottom two as goals from Tyrone Barnett and Alfie Potter gave them a crucial win against rock bottom Tranmere Rovers.
9:30am Saturday 18th October 2014
JOHN Aldridge says he will have divided loyalties when Oxford United and Tranmere Rovers meet this afternoon, but the former striker – a legend at both clubs – is desperate to see them climb away from trouble.
FOOTBALL: Holmes-Dennis becomes United's sixth loan signing as Appleton admits situation is unhealthy
9:30am Friday 17th October 2014
Michael Appleton admits it is “not healthy” to have so many loan players in Oxford United’s squad after Tareiq Holmes-Dennis became the sixth temporary signing yesterday.
9:20am Friday 17th October 2014
JOB security is an abstract concept in football management, particularly at the wrong end of the table, but Michael Appleton insists the only pressure he feels comes from within.