OSMB Oregon State Marine Board suggested the best way to add the "OR" numbers to an inflatable is to make 2 plastic number boards that can be hung on the tube, toon or IK as needed when using a motor.
I've ordered the plastic boards cut to size and delivered from Tap plastics in Portland. I opted for the Polycarbonate white 1/8" boards, look for the website. After you get your OR number from the state, you can lay it out and measure the length you'll need. I went with 5" tall; could have used 4 1/2" measure for yourself to see how you want to order.
For kayak and canoe you may want to contact OSMB to see if they will allow number boards to hang on the bow as opposed to sticking them on a perfectly pristine boat. When they write back print the okay from OSMB and keep it with your boat papers in case an unknowing LEO wants to cite you.