I would start by reading the sticky at the top of this forum.

You can also read the published rules on the oneworld website.

Fares vary primarily based on where you start from, rather than who you buy the ticket from. The airlines do determine taxes/fuel surcharges differently, so there is usually some advantage to having a particular airline issue the ticket (say AA, rather than BA or QF).

If you provide some indication of where you want to go, you'll find lots of people around here willing to provide advice.