Langmead came on as a second-half substitute, and with his first touch, after turning Jon Ashton, he buried a low 20-yard drive into the bottom left corner of the goal.
Glynn Hurst then got a second goal seven minutes from time with a deflected free-kick, awarded for foot up by Leo Roget just outside the box.
And Jamie Tolley made it three in the final minute with a 25-yard drive after some sloppy defending by the home side.
But Oxford, watched by their lowest-ever league gate at the Kassam Stadium, could count themselves somewhat unfortunate because they twice hit the woodwork in the first half.
However, the Shrews also had a penalty superbly saved by Chris Tardif. It came after Hurst was brought down by Matt Robinson, who was lucky to escape with just a yellow card and Tardif then parried Jamie Tolley's spot-kick to his right.
Roget answered an SOS call to become Oxford's captain for the day. The centre-back had been ill with flu for a week, but agreed to get off his sickbed and play after Chris Willmott went down with food poisoning.
The Shrews had two early opportunities to take the lead. Dave Edwards headed inches wide from Neil Ashton's left-wing cross and moments later Hurst wasted a great chance when he broke free on the left edge of the area but struck a weak effort across goal well wide.
Lee Bradbury responded for Oxford with a good effort when he chested down Eric Sabin's knock-down on a run into the box and hit a first-time volley narrowly past Joe Hart's left post.
Sabin frequently troubled the Shrewsbury defence with his blistering pace, giving Stuart Whitehead four yards on one occasion and still flying past him.
Seven minutes before half-time, Tardif made a stunning double save to keep his team on level terms. He first stopped a close-range shot from Tolley, and when Dave Edwards crossed the ball back from the right, Hurst made good contact with his drive, but Tardif somehow got to that as well for a brilliant reaction stop which earned an instant standing ovation from the home fans.
Sabin outpaced the Shrews defence again in first-half stoppage time, and was desperately unlucky to see his curling shot come back off the post.
It was Oxford's fifth defeat in their last six home games.