I believe there is a contractual part of the oneworld agreements signed by all the member airlines that allows for interlining between oneworld carriers, even on separate tickets, as long as the tickets are on the stock of said oneworld carriers.

Having posted that - it is really up to the staff in BRU to do so - they may refuse for several reasons, including with an IB -> IB connection on separate tickets, it's up to IB, not oneworld whether they do so.