What model year?
If this is a 1980's era O'DAY 15 you can get a new rudder from D&R Marine, when you do..add a short leash for it so you won't lose this one

I had one on my Widgeon, the rudder would have sunk if I dropped it!If you have one of the early 1970's era O'DAY 15, then I'd still call Rudy at D&R, but you may need to adapt a rudder for that boat. The 2 different O'DAY 15 models would use a different rudder.