Look and Compare!
The number should be on the transom as stated, behind the rudder. Thats where mine is. I also have a 22 1978. Look close too, because it's just stamped in there. If someone painted over it a few times it might be hard or impossible to see. Does the boat say O'day anywhere on it? Check the pictures here and elsewhere on the net and see if the boats look the same. There's a boat layout line drawing, that will pretty much tell you if it's the Oday.I'm attaching a related link, which will take you to a site that will easily explain how to identify the boat you have. Jensenmar might be right for an Oday. Jensen Marine sticks out in my mind for some reason, but I'm certain someone else can give you more information on that.Good luck!Josh