All of the strand board that I have used for underlayment (o.s.b. or oriented strand board) has a heavy duty waxy finish that not likely absorb the urine into the wood fibres themselves. I have found that it can be exposed to water and hold up much better than uncoated plywood underlayment. I would try these commercial wash treatments, let it dry, and then sniff around. I don't know if paint would stick very well to the waxy stuff. Maybe for good measure you could powder the floor with baking soda and then put a layer of plastic sheeting under the pad just in case?