Hi Vic ...
Well, MAT(T)SON, definitely doesn't seem to be an Oxfordshire surname!
Ann's marriage to Thomas in 1788 is the only female MAT(T)SON recorded on the OFHS's Oxfordshire Marriage Index (1538-1837). The entry gives one small piece of further information - that Thomas gave his residence at Stoke Lyne at the time of his marriage (none given for Ann). It may be worth checking the actual entry in the Bicester parish records to see if the witnesses' names give any clue (or if there is any further information relating to Ann).
Checking the male side of the index, again there's only one MAT(T)SON entry - a Thomas MATSON (widower of Aldgate, London) who marries a widow in Bix in 1778. Although this is a reasonable distance from Bicester, I wonder if this could be Ann's father? In which case, Ann may well have been born in London.
The entry for Ann's baptism that you found on the IGI is submitted, and could well be an assumption made by the poster that Ann was baptised in the parish of her marriage, and she married at the "standard" age of 21. This can be seen time and time again with submitted entries!
With such an unusual surname in the county, it may be worth you contacting the OFHS (
www.ofhs.org.uk) who - for a small fee - will do a search of their Baptism Index. It now covers approx. 90% of the county and you can check which parishes are excluded on their website.
Hope this is of some use!
Pennie