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:
5:34pm Saturday 28th February 2015
4:53pm Saturday 28th February 2015
OXFORD United ground out a valuable point as they held in-form Portsmouth to a goalless draw at Fratton Park.
9:30am Saturday 28th February 2015
JOHNNY Mullins says memories of last season’s terrific win at Portsmouth will be in his mind as Oxford United attempt to pull off a repeat this afternoon.
6:40am Saturday 28th February 2015
MICHAEL Appleton is confident Oxford United’s younger players will be able to handle the atmosphere at a packed Fratton Park today.
2:00pm Friday 27th February 2015
While Gary Waddock’s time in charge at Oxford United was brief, it produced one clear benefit which has helped the side this season.