OLD STYLE

Here is another abbreviation for O.S. - "Old Style"

This is what was carved on Thomas Jefferson's tombstone for his date of birth:

HERE WAS BURIED
THOMAS JEFFERSON
AUTHOR OF THE
DECLARATION
OF AMERICAN INDEPENDENCE
OF THE
STATUTE OF VIRGINIA
FOR
RELIGIOUS FREEDOM
AND FATHER OF THE
UNIVERSITY OF VIRGINIA
BORN APRIL 2, 1743 O.S.
DIED JULY 4. 1826

You see, the Julian calendar was in effect in England and the colonies until
1752. The newly adopted Gregorian calendar, or "New Style", added 11 days
to the Julian calendar to bring it into line with the astronomical year.
Jefferson, apparently wanted posterity to know his actual birth date
according to the calendar under which he was born, which was the "Old Style"
or Julian calendar.

http://www.monticello.org/reports/life/old_style.html