Your boat is too old
The link to O'Day Sailboats is great, but won't help for a boat older than 1973. For boats built before the 12-digit HIN was used there will be a metal plate (located usually in the cockpit) that has 2 numbers on it. The first number is a "HULL #" the second is a "CLASS #", hull# refers to total O'Day sailboat production, Class# refers to total for that model. To use my old Widgeon as an example: Hull# 22698 Class# 1791, she was the 22698th O'Day built and the 1791st Widgeon. To determine what year a boat was built is hard with these numbers, if O'Day was still in business and still had the records, a call or letter to them might give you an answer. However, since O'Day is no longer in business, you can only estimate when your boat was built by comparing notes with other O'Day owners that have confirmed the build year of their boat.