Original: The notation "Old Style" (OS) is sometimes used to indicate a date in the Julian calendar, as opposed to "New Style" (NS), which either represents the Julian date with the start of the year as 1 January or a full mapping onto the Gregorian calendar.
Simplified: People sometimes use the term Old Style or O.S. to refer to the Julian calendar, with N.S. or New Style referring to the Gregorian calendar.