As
wannaflyforless states clearly (and
oneworld and American Airlines access rules do too,) the outbound flight must be operated and markleted by a
oneworld airline for access to a
oneworld lounge. Codeshares mean absolutely nothing and are not eligible for lounge access, other than for the occasional mistaken admission.
In this instance, if the OP were on a B-Flex fare, there would be access to the Brussels Airlines lounge in the Schengen terminal. On arrival at the north Zaventem terminal, the OP will have to go through the BRU equivalent of flight connections, no hope of using the BA lounge, nor would the OP be granted access to use it as an arrivals lounge.
Links to both policies are in the
Unofficial Users' Guide, but I will provide them (yet) again.
Admirals Club Lounge Access Policies
oneworld lounge access policy
which states, in part:
"
Access to Airline Lounges
The oneworld® alliance airlines provide more than 500 lounges across the globe. If you are an Emerald or Sapphire level top-tier member of one of our frequent flyer programmes, you will have access to any of these lounges before departure, no matter which oneworld airline you are flying. Lounge access is available if the departing flight from that airport is marketed and operated by a oneworld carrier. Access is subject to lounge capacity."