Any oneworld member carrier can issue any oneworld fare product. It is allowable under the rules for IB or AA to issue a xASnn, even though they have no flights within the area the ticket allows.

The xONEx online booking tool takes the marketing carrier of the first flight as it is easier for the programing to work it out. Some carriers (eg, QR) have opted out of ticketing via the tool, others (eg, JL, RJ) have elected for other carriers (mainly AA) to ticket for them.

The main issue is that some carriers (such as AA) don't want to ticket an itinerary where they don't have their code on some of the longer flights such as the intercontinental or inter-regional flights.
Others (such as CX) think that the technical limitation of the online tool booking to the first marketing carrier is an actual ticketing rule and have been known to refuse even looking at the booking if their code is not on the first flight (I have had to quote fare rules to CX a number of times to get them to issue quotes and finally ticket trips).

There is no rule preventing any oneworld carrier from ticketing any oneworld product. Actually getting them to do so can be a whole other matter.