Thanks everyone for the help. I called the SAS lounge at ORD and they informed me that OW Emerald/Sapphire elites traveling on Cathay do have access to their lounges. All they need to do is go up to the Cathay desk and get your boarding pass stamped. I guess I'll do that.

I could also use the AA lounges but they're not in terminal 5, so it would be a bit more difficult.