The fact that your boat has a Hull# and a Class# indicates that she was built prior to the November 1, 1972 USCG Hull ID Number requirement, so we know she is a 1972 or older model year.
Hull# refers to the total number of boats built (all models) by O'DAY up to the time of your boat (you have the 3015th boat built by O'DAY) Class# refers to how many of that model, so you have the 333rdboat of that model (Class).
Now, the first problem, that low of a hull number indicates a pretty early O'DAY, like 1960 or may be '61 or '62. O'DAY built 2 different models designated "O'DAY 15" and the earliest version was built in 1970-73, later version, and completely different boat, was built in 1981-83. So, your boat most likely is not an "O'DAY 15".
O'DAY did build a 15'8" boat, known as the OSPRAY starting in around 1960, that boat was more or less a smaller version of the O'DAY Day Sailer, sort-of, but not quite. Realistically, any boat built prior to the 1973 model year can not really be precisely dated. I guess whether she is a 1960 or 1961 doesn't make that much difference at this point....... but, the exact model year is not easy to detemine.
Anyway, below you will see several pictures of OSPRAYs, is that what your boat looks like?
