Seychelles is actually in the Indian Ocean, and is served by BA.

Qantas used to serve Tahiti, but dropped the service in preference for only running the codeshare. No dice on an OWE. Lan Chile technically serves Tahiti, though it would be a real big pain to actually use them because you would need to find your way SYD-PPT first.

You are correct that the Oneworld explorer leaves a lot to be desired in the South West Pacific islands. The Global Explorer, however, is good because you can also use Polynesian Airlines and Fiji Air. If I am not mistaken, with the OWE, you are limited to New Caledonia and Papua New Guinea.