Last year I booked same flight and was told I could use Ba or Emirates, so I was wondering if I could use cathay
EK5102 is actually operated by Qantas. If the boarding pass says Qantas (QF2) and you’re in Business Class, irrespective of oneworld status, you can probably get into the CX lounge.
If it says EK then, as others have suggested, you may not have any luck. Good luck!!