Having enjoyed the better of the opening period, the Magpies were slow to come out of the traps after the break, and were punished within seconds, as Kane Hemmings struck from close range.
Rafa Benitez's side then spurned a glorious opportunity to equalise in the 65th minute, as Aleksandar Mitrovic’s penalty was turned wide by Oxford’s Simon Eastwood.
And, a quarter of an hour later, Curtis Nelson put daylight between the sides after meeting Rob Hall’s looping corner.
Debutant Toni Martinez rubbed salt into the Magpies’ wounds late on, heading beyond Matz Sels from another Hall delivery.
As a torrential downpour quickly gave way to bright sunshine, the home side had made a positive start. Less than five minutes in, they were awarded a free-kick in an inviting position 20 yards out. Chris Maguire stepped up to take, but could only fire straight into the wall.
United’s first sight of goal fell to Yasin Ben El-Mhanni. After Isaac Hayden’s long-range strike had deflected into Jesus Gamez’s path, the Spaniard’s looping cross was met by the young winger at the back post, but Eastwood gathered with ease.
Midway through the half, El-Mhanni took aim from 20 yards after cutting in from the left, but saw a low-struck effort trickle wide via the leg of Oxford’s Phil Edwards.
Just before the half-hour mark, the U’s came close to opening the scoring. Maguire and Hall exchanged passes after a short corner, and the latter drew a fine save from Sels from a tight angle.
The Magpies then threatened twice in quick succession. First, Gamez saw a rising first-time effort held by Eastwood, before the ‘keeper produced a smart parry to deny El-Mhanni.
Eastwood was called into action again in the 35th minute. Mitrovic latched onto Ayoze Perez’s through ball, but the stopper narrowed the angle and diverted the Serbian’s effort wide.
Mitrovic perhaps ought to have broken the deadlock with the last action of the first half. Gamez released Perez down the right, who pulled the ball back for the frontman, well-placed just inside the area. However, Eastwood again came to Oxford’s rescue with an instinctive stop.
But, within seconds of the restart, Newcastle found themselves trailing.
They failed to clear their lines following a corner from the right, allowing Maguire – the original taker – to whip in another inviting ball. His delivery was nodded down by Chey Dunkley at the back post, and Hemmings was on hand to scramble home from barely two yards out.
Looking to respond, Grant Hanley – the Magpies’ skipper for the day – flicked Achraf Lazaar’s corner high of the target 54 minutes in.
Hayden then nodded straight at Eastwood following another Lazaar corner, before Dan Barlaser skewed wide from 20 yards.
Midway through the half, the visitors squandered a golden chance to restore parity.
Though Mitrovic did superbly to burst into the box and draw a foul from Edwards, his spot-kick lacked placement, and Eastwood duly pushed the ball wide.
With a quarter of an hour remaining, only a superb Hanley challenge prevented Hemmings from netting a second, the forward having looked set to convert following Hall’s knockdown.
But, five minutes later, Oxford all but made sure of their place in the last 16.
Nelson was their marksman, rising high at the back post to plant home Hall’s corner from the left.
It got even worse for the Magpies late on.
Hall was again the architect, delivering another fine cross – this time from the right – for Martinez to glance home.
Oxford United: Simon Eastwood, Joe Skarz, John Lundstram, Curtis Nelson, Chris Maguire (Toni Martinez 70), Kane Hemmings (Liam Sercombe 83), Phil Edwards, Rob Hall, Ryan Ledson, Marvin Johnson, Chey Dunkley.
Newcastle United: Matz Sels, Jesus Gamez, Grant Hanley, Curtis Good, Massadio Haidara (Matt Ritchie 77), Isaac Hayden, Dan Barlaser, Achraf Lazaar, Ayoze Perez, Yasin Ben El-Mhanni (Yoan Gouffran 68), Aleksandar Mitrovic.
Referee: David Coote
Bookings: Oxford United – Kane Hemmings (58), Phil Edwards (65); Newcastle United – none.
