Current practice is that most OW airlines will not price and issue tickets unless they are flying one of the 3 money sectors (trans-atlantic, trans-pacific or trans-asia). You probably could not get AA to issue you that ticket today unless you had an AA trans-ocean leg. The rules allow any OW airline to issue any OWE ticket, but the only one forced to issue it is the airline with the first flight (in this case BA until the AA codeshare showed up).