Welcome to another Jez!
Apologies if I misinterpret your query.
To qualify as a OWA in addition to other rules all the flights have to be booked on the one PNR, the booking reference which is a combo of 6 letters & numbers.
You book the first flights online or over the phone & then as other flights become available you will have to phone up & get them added to your original PNR. You cannot make separate bookings (different PNRs) & combine them at a later date.
Below is a link to an AFF article about the OWA which gives some great info & this thread has a lot of useful info & alerts to some of the hurdles being encountered eg flight changes, ticketing delays etc
Fly around the world with Qantas and other Oneworld airlines by redeeming points for a Oneworld Classic Flight Reward. Photo: Qantas. Did you know that
www.australianfrequentflyer.com.au