I always assumed the answer was that it was because your guest also had to be flying on a oneworld carrier, but I thought I'd chance it in CPH today and see if my colleague who was flying SAS could get in as my guest to the lounge.

They scanned my colleague's boarding pass barcode which presumably shows them which carrier etc. in the computer system so I can only think that the lounge receptionist either ignored the policy about them flying on oneworld flights or that it isn't the policy. If it isn't the policy that you have to be flying oneworld to be guested into the lounge, why do they check the boarding pass?