Police found the child after being called to at a home at Bromsgrove Cottages in Faringdon at around 13:20 GMT on Tuesday.
A 40-year-old woman has been arrested on suspicion of murder and the child's next-of-kin have been informed.
The boy was said to be of primary school age. Police have increased their presence in the area.
They said this was in order "to provide reassurance".
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Supt Rory Freeman, local police area commander for South and Vale of White Horse, said: "This is a very shocking discovery and I am aware it may have a very direct impact on the community of Faringdon.
"We would advise any members of the community affected by this incident to speak to one of the neighbourhood policing team."
The woman remains in police custody. Anyone with information is asked to call 101 quoting 730.
Police tape remains around the line of cottages, and uniformed and plain clothes officers are coming and going as the investigation continues.
News that something serious had happened spread rapidly on social media on Tuesday night.
This morning passers-by are saying they are shocked and saddened by what's happened.
One lady walking to school with her son stopped to give a rose to one of the police officers monitoring the cordon.
This is quite a visible place opposite the fire station; a lot of people go past here, so the police are asking anyone with information to contact them.
A murder investigation has been launched after a boy was found dead at a house in Oxfordshire.