When you book with a OneWorld airline you can view your booking on any other Oneworld airlines website. Except for funnily enough JAL.
If you’ve booked a mixture of Qantas and JAL sectors the best way to change your seats for the JAL sectors is to view your booking on the Cathay Pacific website.
This will allow you to select a seat preference (Window or Aisle) then as soon as you save it, JAL will allocate you a seat. If you don’t like it change it to Aisle then back to Window and you will sometimes get a different spot.
If you really need a specific seat you can call JAL and ask them to move you. But usually, the above workaround gets you what you need.