Originally Posted by
Andriyko
Apart from the obvious capacity explanation (that lounge can barely accommodate F passengers), AY may simply not want to pay for access to BA lounges in addition to the negotiated access to AF/KL lounge.
Whether or not AY is happy to pay BA for access to their lounges is irrelevant here- any Oneworld Carrier lounge is available to Oneworld carrier status or J/F passengers irrespective of the airline flown. The exceptions are BA CCR, QR lounges, AA lounges for AA members flying domestically.
The reason the OP was turned away from the BA lounge was due to capacity limitations, which is also within the Oneworld terms and conditions of lounge entry.