This post probably stems from my ignorance of what airline alliances actually mean but I was looking at the Qantas website and noticed that their route maps and bookable city lists didn't show all the options available if you included other oneworld member airlines. Take, for example, Helsinki, the home port of oneworld member Finnair. Neither can you see Sydney from the Finnair website. Likewise, you can't see flights from Iberia and JAL, other oneworld members.
A look at the oneworld route map gives you a good visual example of the potential options. I can see why you couldn't easily list all possible oneworld destinations on a simple drop down list on the individual airline websites, although, as I web developer I can think of methods to do so. Isn't a major point of such alliances to open up destinations not served by one airline and to offer a fairly seamless way of transferring between the allied airlines? I haven't tested Skyteam or Star Alliance to see if the situation is the same.
If I were to call an airline's booking service directly, would they easily (and willingly) place me on a co-member's flight, especially on a non-competing route? In this age of the efficiency drive I do find it surprising that you can't do it on the individual airline websites.