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.
The body of a man has been found at a property in Oxford, police have confirmed.