Oceania is a geographic region that encompasses most, if not all, land that is not covered by the five "main" continents (plus Antarctica). It is considered a "continent in name only" by a lot of people, for the sake of dividing the world's nations into six easy categories.
Australia the continent is officially the "sixth" continent, and school children in the 90's were taught a song that identifies "North America, South America, Africa, Asia, Europe, Australia, Antarctica". Though Australia the continent pretty much only contains Australia the nation and New Zealand (and is therefore sometimes called Australasia), people in the past tended to rope in all of the Pacific island countries that didn't have a continent otherwise.
So, though Oceania is not a continent, it is used in place of one to make things a lot less confusing.