An 84th-minute penalty by Simon Weatherstone proved enough to keep Eastbourne in the Blue Square Premier in a nail-biting relegation decider.
Boro faced high-flying Oxford knowing a win should ensure their survival.
But after Luke Rooney struck the underside of the bar and Weatherstone twice came close it looked like Boro could be dropping down.
But when Mark Creighton handled Luke Benjamin's shot in the Oxford area Weatherstone made sure from the spot.
The home side enjoyed the majority of possession before the interval but could not break down a resolute Oxford defence.
Midfielder Weatherstone had two good chances in the opening half as he first met a cross from the right with a powerful header which flew wide.
His second came in the final minute of first-half extra time when a sweeping move finally found the Oxford defence stretched.
Weatherstone unleashed a powerful strike from inside the area that beat Ryan Clarke in the U's goal but sailed wide of the right upright.
The home side upped the pace after the break and took the game to Oxford.
Creighton came close with a close-range header off a Jamie Cook corner after two minutes.
But Rooney had the Priory Lane crowd out of their seats on 55 minutes when his shot hit the under side of the crossbar only for the ball to spin back out.
Oxford recovered well and managed to pin back their hosts back as both sides cancelled each other out.
The U's sustained their pressure with just 10 minutes on the clock and John Grant's powerful shot but was pushed wide by the stand-in Boro keeper Greg Nessling.
As the clock ticked down, and with results elsewhere pointing to Boro's relegation, Ben Austin tried to seize the initiative by drilling a 30-yard shot which went inches wide.
But then Creighton's intervention handed Boro the lifeline that they needed.
Weatherstone stepped up and drilled his right-footed penalty past Clarke to ensure the Sussex club a third successive season in the Blue Square Premier.
Nessling, Baker, Austin, Elphick (Johnson 73), Brown, Rooney, Weatherstone, Smart, Matt Crabb, Atkin (Enver-Marum 34), Taylor (Benjamin 65).
Subs Not Used: Pullan, Nathan Crabb.
Goals: Weatherstone 84 pen.
Oxford Utd: Clarke, Batt (Chapman 61), Creighton, Hargreaves, Sandwith, Day, Chalmers, Deering, Tonkin (Clist 71), Grant, Cook (Matthew Green 85).
Subs Not Used: Turley, Woodley.
Booked: Chapman, Matthew Green.
Ref: Brendan Malone (Wiltshire).