Thames Valley Police made the discovery after being called to the property in Iffley Road at 14:58 BST. They are treating the death as "unexplained".
The force said Iffley Road would be closed to drivers and pedestrians for a "number of hours" between The Plain and Circus Street.
Officers are appealing for information from the public.

Cooper, 18, was registered with the National League in time for him to be part of the squad for Saturday's home fixture against Braintree Town.
The son of former Swindon, Peterborough and Darlington manager Mark Cooper, the Blues under-21 player signed a first professional deal last summer.
However he is yet to make a senior appearance for Birmingham City.

Trainer John Gosden let the unbeaten 4-7 favourite race after he walked the course and she beat runner-up Vintage Folly by a length and three quarters.
She goes to Chantilly next rather than the Oaks at Epsom, in which stablemates Enable and Coronet are top contenders.
Tasleet (14-1) took the Duke of York Stakes for trainer William Haggas.
Jim Crowley's mount won comfortably from Magical Memory in second, with Comicas third.
Shutter Speed, whose last run at Newbury saw her beat Enable into third, brought a fifth Musidora win for both Gosden and Dettori.
"We got away with it. The ground was very soft. It was hard work for her," said Dettori.
"She'll be a better horse on better ground. She's top drawer. Everything she does is very classy."
Gosden is concerned about the ground for his colt Cracksman in Thursday's Dante Stakes and will again walk the course before deciding whether he runs in the Derby trial.
BBC Sport horse racing correspondent Cornelius Lysaght
Shutter Speed's clearly in very good order and she coped well with the soggy conditions.
She'll be off to France's Oaks, staged over 'her distance' of a mile and a quarter, and Gosden will rely on Enable and Coronet in the longer Epsom Oaks.
The trainer said he was pleased to now have a month to prepare Shutter Speed for Chantilly.
Meanwhile, the gap between Thursday's Dante Stakes and the Derby is just 16 days, so Gosden's colt Cracksman may not be asked to line up in what's sure to be an examining Dante, in terms of both quality and going.
The body of a man has been found at a property in Oxford, police have confirmed.
[NEXT_CONCEPT]
Forest Green Rovers have signed midfielder Charlie Cooper from Birmingham City on a one-month loan.
[NEXT_CONCEPT]
Shutter Speed overcame rain-softened ground to win the Musidora Stakes at York under Frankie Dettori and will now be aimed at next month's French Oaks.