It is not surprising that you cannot find “Oceania” in Guns, Germs, and Steel.” A quick scan of my digital copy shows that the word is used only in the notes. What is going on is that Diamond does not talk about Oceania as the scene of the natural experiment. Instead, he talks about Polynesia, which is part of Oceania. Polynesia was a good site for a natural experiment because it had people from a single culture colonizing islands with very different geographical characteristics. Diamond discusses this in Chapter 2 of the book.
Diamond feels the need to conduct this experiment because many people tend to think that culture determines how successful a society is. Societies succeed (they say) because their cultures are more suited to progress than the cultures of people who are weaker and poorer. Diamond says this is not true. He says that geography determines whether societies become rich and strong. Since Polynesia had many different kinds of islands colonized by people who shared a single culture, it makes a good site for a natural experiment. You can hold the variable of culture constant and change the variable of geography. If you end up with societies that are the same, you know geography does not play a role in determining success. If your societies end up very differently (some rich and some poor, for example) it will support the idea that geography is much more important than culture. This is why Oceania (or at least Polynesia) was a good place to undertake the natural experiment.