Obiter's post set my mind wandering. Two favourite Os. Firstly Madame Roland's famous "O Liberte, que de crimes on commet on ton nom". Secondly the wonderful Robert Hardy's Latin master explaining the declension of the noun mensa to a bemused young Churchill (in the superb film of that name): "Mensa with a long A , Churchill. Vocative, as in addressing a table : "O, table!".