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:
9:30am Friday 19th September 2014
TOM Newey hopes Oxford United’s supporters will embrace the side’s progressive style after seeing them make it work for the first time at the Kassam Stadium.
9:20am Friday 19th September 2014
HIS varied CV may include a spell at arch rivals Swindon Town, but Brian Howard’s debut for Oxford United should have gone a long way to convince any doubters not to hold it against him.
9:00am Friday 19th September 2014
DEREK Fazackerley says he has been impressed by the way Michael Appleton has dealt with a challenging start to life as Oxford United’s head coach.
8:50am Friday 19th September 2014
MICHAEL Collins says he was moved by the support from Oxford United’s fans and staff during a difficult time earlier this month.
8:40am Friday 19th September 2014
GRAHAM Westley believes Oxford United’s new signings will help to make life difficult for Stevenage tomorrow.