Ansaf Azhar said the rising problem had been exacerbated by falling activity levels since the pandemic.
The Oxfordshire County Council health director said those most affected were from deprived areas.
Mr Azhar said he was working to make physical activity easier to access, particularly for children, as well as providing education about health food.
The average for England is 64%.
Mr Azhar said: “This has the potential to cause a bigger health impact than the Covid pandemic itself,” but added that it would over a much longer time period.
“If you have a generally iller population, that’s not good for the residents, it’s