John R Wightman married Annie D Proudlock in Woodstock 1913

What specifically do you need from the marriage certificate? You could either go to the Oxfordshire Record office and look at the parish registers there, or you could order the marriage certificate from GRO for £9.25, they arrive usually within a month, sometimes within the week.

I found a death for John possibly last year? If you search his name into Google, it comes up with a funeral service for a man of the same name.

There was a John Raymond Wightman born in 1898 in Witney. That would make him only around 16 when he married though, and I'm not too sure if you could marry under 18 in those days. The marriage certificate would tell you, as it would say whether he was a minor.