The visitors were in charge of the first round tie for most of the afternoon, but were unable to take advantage until the second half.
Constable headed the opening goal at a corner before laying on the second for Rigg, who had hit the crossbar before the break.
It was comfortably enough to see off an unambitious Barnet and seal the club's first win in the FA Cup for three years.
United made two changes to the side which started the 3-1 win at Wycombe Wanderers a week earlier.
Michael Raynes replaced the cup tied Johnny Mullins, while Jon-Paul Pittman came in for Tom Craddock, who dropped out of the squad altogether.
The substitutes included 16-year-old striker James Roberts, promoted following a goal streak for the youth team.
Barnet, bottom of npower League Two, set out to frustrate the U's from the kick-off with a five-man midfield.
It allowed United to control much of a first half in which they were on top but failed to make a breakthrough.
They almost took the lead with a fluke on eight minutes, when Andy Whing's misdirected cross hit the top of the crossbar. On the rebound Constable had a shot superbly blocked.
The striker turned provider on 19 minutes with a surge into the box. His cut back was perfect for Alfie Potter, but the winger fired wide from close range to waste a glorious chance.
It took the Bees half an hour to register any sort of attempt on goal - an Andy Yiadom shot from 30 yards which flew well over.
The only time United's defence looked flustered came eight minutes before the break.
Collins John got in behind the back four and squared for Clovis Kamdjo, whose finish was awful, flying well wide.
Rigg nearly broke the deadlock in first half stoppage time, but his vicious strike from 16 yards hit the crossbar and bounced clear.
The interval revived Barnet, who went on to enjoy a decent spell without troubling U's goalkeeper Ryan Clarke.
It was only a brief diversion and United quickly re-established their authority.
On 56 minutes the visitors finally took the lead when Constable powered in a header from a Leven corner.
It was the striker's fifth goal in five games for United against Barnet and he came close to adding another just after the hour. Collecting the ball on the left he cut inside and curled a shot just wide.
Lee Cox replaced the ineffective Pittman for the final half hour, as Potter was pushed forward.
The game was there for the taking, as United created chances almost every time they went forward.
Young goalkeeper Sam Cowler showed good reactions to twice deny Rigg, who should have done better.
The winger made amends with the crucial second goal ten minutes from time.
He owed much to Constable, who controlled the ball superbly and played in an immaculate pass for Rigg to sprint on to and coolly finish.
United comfortably saw out the closing stages to clinch a straightforward victory.
Oxford Utd: Clarke, Whing, Raynes, Wright, Capaldi, Potter, Chapman, Leven, Rigg, Pittman (Cox 63), Constable.
Unused subs: Brown, Worley, Heslop, O'Brien, Roberts, Marsh.
Barnet: Cowler, Fuller, Stephens, Fortune, Brown, Kamdjo, Byrne, Lee (Davids 73), Weston (Allen 73), Yiadom, John (Lowe 61).
Unused subs: O'Brien, Saville, Abdulla, Edgar.
Referee: Tim Robinson.
Attendance: 2,346 (834 visitors).