Could you post a picture? a 12' O'DAY could be a WIDGEON (sloop rig, main and jib) or an O'DAY 12 (cat rig, single large sail) I'll post a pic of each below. If you have a WIDGEON, then I do have an Owners Manual that I can send a copy of (it is expanded from the original, so I do have to charge $10 for printing and postage, but wil linclude color scans of various Widgeon brochures). I used to own a 1969 WIDGEON so can answer questions about the Widgeon, I'm basically familiar with the O'DAY 12 ,but answers will be based more on pictures and 1978 sales brochure.
The WIDGEON and O'DAY 12 were basically the same hull, but different decks. The Widgeon had a pivoting fiberglass CB, the 12 had a wooden daggerboard that was raised/lowered through a vertical trunk (thin box-like enclosure). Widgeon has wire standing rigging to support the mast, the 12 had a freestanding mast.
If you are missing any parts, I'll second Joe's advice to call Rudy at D&R Marine!
PS: The emblem on a WIDGEON sail is a "W" surrounded by an "O" and the O'DAY 12 has a "12" surrounded by an "O"