Some 82% of money generated last year went to the richest 1% of the global population while the poorest half saw no increase at all, the charity said.
Oxfam said its figures - which critics have queried - showed a failing system.
It blamed tax evasion, firms' influence on policy, erosion of workers' rights, and cost cutting for the widening gap.
Oxfam has produced similar reports for the past five years. In 2017 it calculated that the world's eight richest individuals had as much wealth as the poorest half of the world.
This year, it said 42 people now had as much wealth as the poorest half, but it revised last year's figure to 61. Oxfam said the revision was due to improved data and said the trend of "widening inequality" remained.