The first you list, with plastic, semi-diamond shaped flange, is used on the transom of my Force 5. I also installed the same in my skin-on-frame stand up paddleboard build. So far, the Force 5 has been dry, but I do need to drain the SUP after a paddle as there is some seeping after 3 coats of paint on the polyester fabric.
The second one you list is typically found on powerboats. When someone says "Always remember to put your plugs in before you launch," they mean this type of plug. We had 2 of these plugs under the back bench seat, and under the motor well, in Granddad's 1963 plywood lapstrake 19' runabout. He told me the story of forgetting to put them in prior to launch one year, when his back was "out," and his friend was aboard. They motored slowly away from the ramp, and out onto the bay, when the friend noticed water coming up the floorboards from under the seat. Granddad said he dove under the seat to install the plugs, and his buddy had to drag him back out by his feet, because he couldn't move after aggravating his bad back.
The third one you list I am not familiar with, though it says it's a water outlet. I imagine it would be used as a deck fill for an internal water tank.
I would be surprised if any of these were used on your O'day 20. There are certainly none on my 192. The only thing I need to worry about is the hose between the transom through hull fitting, and the same fitting at the aft end of the cockpit. If this hose deteriorates, I could leak water from the outside directly into the cabin, or rain water from the cockpit straight into the cabin. My friend had to replace his last year while his wife and guest sat up on the bow and chatted, keeping the transom drain out of the water while he raced to the hardware store to get fiber reinforced vinyl tubing to replace it. The old hose looked very much like wire reinforced sewage hose used in the head systems of larger boats.
