Britain is one of the most unequal countries in the developed world and contributed to the vote for Brexit, the charity said.
The richest 1% of the UK population owns more than 20 times the wealth of the poorest fifth, according to Oxfam.
Its analysis found that about 634,000 Britons were worth 20 times as much as the poorest 13 million people.
Oxfam has urged Prime Minister Theresa May to help close the gap between the “haves” and the “have-nots”.
The charity’s report analysed data from Credit Suisse and found that the richest 10% of the UK population own over half of the country’s total wealth, with the top 1% owning nearly a quarter (23%).
The poorest 20% share just 0.8% of the UK’s wealth between them.
The report said many people in the UK felt locked out of politics and economic opportunity.
“Whatever your views on Brexit, the referendum brought divisions within our country to a head, with many people expressing distrust and disconnection with political processes and voting for change in the hope that it would improve their economic position,” Oxfam said.