Getting to ATH requires using either BA or IB while to get out of SIN requires one of CX, QF, BA, AY ... so no problem qualifying for the Oneworld award (and the requirement is actually to fly 2 OW airlines other than AA, flying AA is optional). As for the original question about the open jaw, my understanding is that the open jaw distance is included in computing the mileage needed for the award -- so there is no advantage to open-jaw rather than fly it on OW awards which allow stopovers. While on other awards the open jaw is a way to get another stopover. A clever idea to use the open jaw to fall below an award tier boundary, and it may work on some airline FF plans but I think for AA the rules allow it but include the mileage in the award cost.