So, just asking out of curiosity: anyone know how airlines in the alliance offer benefits for members of a partner's FFP? Naturally, there are agreements in place and they must have seen some benefit in signing them (e.g. enticing members of other FFPs to fly with them; or to make their own FFP more valuable to consumers). But really, who pays for the oneworld Priority benefits? The operator of the flight or the passenger's programme airline?

It seems to be a mix of both. Such as lounge access, it seems to be that operating airline pays based on class of service, and if you access a higher lounge based on status, the FFP pays.
eg
QF EMB flying JL J going to at CX F lounge, QF pays.
BA EMB flying CX F going to JL F lounge, CX pays.

My understanding is that this is how *A works. For OW, the operating airline always pays the lounge fee. When I was a CX SPH member flying AA Y going to the BA lounge at BWI, I remember the lounge complained about not having the payment scheme set up with AA, the only other OW carrier at BWI. If BA were supposed to get money from the airline I have status with, I'd assume the BWI lounge need to have the scheme in place with every single OW carrier.

You are absolutely right,in oneworld it's the operating airline that pays for access,whereas in Star Alliance and Skyteam it's the FFP which pays. Lounge fees depend on the airline and location (and the range is very high).

I entered the DFW D AA lounge on Monday flying QR J. The AA agent scanned the QR pass, then swiped my QF Plat card, then gave me the invite into the Flagship Dining area (an F level benefit). There would have been no reason for the lounge agent to swipe the QF card, which was already in the booking, if the QR pass was good enough for access.

Yes,because AA has to explain to QR why they (AA) let a particular passenger to the lounge. Sometimes they swipe the card,sometimes type the info in,sometimes write it down.

This seems inconsistent for ST, where DL no longer provides "third party" lounge access (ie: those not operated by a ST airline), but DL STE+ report access to such lounges when flying on non-DL metal (ie: operating carrier would be paying).

Himeno is correct. (although there are perhaps nuances and differences depending on airline and location - I can't speak for all).

But at the OW airlines and lounges that I am familiar with - operating carrier pays when access is granted based on COS and FFP pays when access is granted based on FFP Status.

So I assume you have info from senior OW airline managers,who lied to me?

Yes - for the lounges and arrangements that I'm familiar with - my info comes from the senior airline execs themselves.

Now - that doesn't mean that other airlines and lounges within OW have different arrangements.

It also doesn't mean that there aren't differences between standard lounge access (such as Admirals Club or Business Lounge) vs premium lounge access (such as Flagship Lounge or First Lounge).

Also - getting off the oneworld-specific discussion....

Often it is actually the marketing carrier (as opposed to operating carrier) that pays for lounge access.

Additionally - many airline partnership agreements are assumed 'traffic-neutral', in which case - in many instances there is no transfer payment for benefits/lounge access. This varies depending on traffic imbalances and risk-aversion for the particular carrier.

So the short answer - is that like many things - "it depends".