O.S.

Old Style (O.S.) and New Style (N.S.) are terms sometimes used with dates to indicate that the calendar convention used at the time described is different from that in use at the time the document was being written. There were two calendar changes in Great Britain and its colonies, which may sometimes complicate matters: the first change was to change the start of the year from Lady Day (25 March) to 1 January; the second was to discard the Julian Calendar in favour of the Gregorian Calendar. Closely related is the custom of dual dating, where writers gave two consecutive years to reflect differences in the starting date of the year, or to include both the Julian and Gregorian dates.

See Wikipedia:Old_Style_and_New_Style_dates