Sure. The OW booking tool will revert to the airline operating the first segment (with some exceptions) that doesn't mean that you have to do that if you aren't using the tool. You cannot can book with any Oneworld airline that you want (and that will agree to book and issue the ticket for you). Doesn't matter who the first leg is on. There are any number of us that have booked through AA and then flown to LHR on our first intercontinental flight. Of course that said, at times AA has balked unless at least one of your long-haul overwater segments is on AA metal.