Two questions:
If I "buy" a flight using miles, will I earn miles on that flight? In my case, I'd be using American Airlines miles to buy an American Airlines flight.
What about if I use American Airlines miles to "buy" a flight on some other oneworld carrier, like Japan Air Lines? Will I earn JAL miles or AA miles?
No, you don't!
From the terms and conditions of American Airlines AAdvantage program:
You may accrue mileage only for purchased, eligible, published-fare tickets on qualifying routes used in accordance with all applicable conditions of carriage, tariffs, rules and terms of ticketing and travel. AAdvantage mileage accrual eligibility on airline participant routes is subject to change without notice.
and even more clear:
Certain airline tickets are not eligible for earning mileage credit. These include, without limitation, the following: all tickets issued as AAdvantage awards or other free ticket promotions including free or reduced rate tickets;...
As you don't get new mileage on AA, you most certainly also don't get any on any other airline.
