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:
4:14pm Thursday 2nd July 2015
RYAN Clarke has admitted it is a sad day but he leaves Oxford United with "amazing memories" after six years at the club.
1:00pm Thursday 2nd July 2015
OXFORD United have received a major financial injection with lifelong supporter Stewart Donald set to acquire a ten per cent stake in the club.
9:30am Wednesday 1st July 2015
GEORGE Baldock described the decision join Oxford United as a “no-brainer” given the determination the club had shown to bring him back.
4:00pm Tuesday 30th June 2015
GEORGE Baldock has agreed to return to Oxford United on a season-long loan deal from MK Dons.
12:00pm Tuesday 30th June 2015
OXFORD United’s game at Bristol Rovers in September has been selected for live coverage by Sky Sports.