The child was in the Hugo Boss shop at the Bicester Outlet shopping village in Pingle Drive on Tuesday when it is believed part of a shop fitting fell on him causing serious head injuries.
He was taken to the John Radcliffe hospital in Oxford where he died in the early hours of this morning, Thames Valley police said.
Investigating officer Detective Inspector Steve Duffy said: "Our thoughts are with the family at this time.
In a statement, Hugo Boss said: "At around 8.30pm on June 4th, a four-year-old child was injured by a mirror falling on him. The child suffered serious injuries and was treated at hospital where this morning he passed away.
A company spokesman refused to reveal the size of the mirror or give details of how it fell and whether it fell from height.
He said the store would remain closed until further notice.
The Health and Safety Executive said that they were not involved in the investigation into the death of the boy as shops are the responsibility of local authority environment health department