Pretty much. Just have a bit of paper-towel or something when you unhook the line to plug the hole with. It won't leak very much. You don't need to "refill" the oil cooler. The oil pump will do that within a few seconds after you start it up. The oil from the pump goes to the oil filter first, then to the oil cooler. After the oil cooler the oil goes to the crank and transmission. It will actually take less time to pressurize the system than when you replace an oil filter.
Thanks, crash. Changed the o ring yesterday, and it seems fine now. Had a bit of oil come out, as you explained. Havent topped up as not sure what the previous owner put in - he only changed the filter/oil via the mechanics for the RWC 2 months back.