Ok - I have now had time to look a bit further - I will take back what I said up there

There are only 2 mentions of the surname Oliver in the Charlbury registers beween 1740 and 1800, the marriage already mentioned and a baptism in 1779 of Thomas s/o William & Mary Oliver of Finstock, there is no baptism for a Mary Oliver of Finstock around the time we would be looking.
From the Oxford Marriage index there is a marriage in 1775 (26th Sept) in Stonesfield, which is very close to Finstock of William Oliver to Mary Hancock of Finstock. I could not see a burial for William in the Charlbury registers which cover Finstock - but it could possibly be in Stonesfield and I don't have those registers.
There is a baptism in the Charlbury registers
1747 4th Nov Mary House Hancock d/o Elizabeth Hancock of Charlbury base born which would fit with the dates given. So I will have to backtrack and say it is quite probable that Mary was a widow when she married Joseph Launchbury, though it would be more convincing if a burial for her first husband William Oliver could be located.
DebbieG