The Medieval Latin abbreviation for Oxford or Oxfordshire. It is used both in the abbreviations of university degrees, e.g. B.A. (Oxon), and as the name of the county in addresses, e.g.
32 Railway Cuttings
Great Trumpington
Oxon.
The Latin name is Oxonia, which may be seen in the adjectival form Oxoniensis, e.g. in titles of old publications.
Compare with Cambridge, which uses Latin Cantab for the university but an ordinary English abbreviation Cambs for the county.