Australasia and Oceania are not the same thing, but they are in relation to one another. Oceania is a continent which is home to Australasia. Australasia is made up of the countries Australia and New Zealand. It also contains neighboring islands, such as islands in the South Pacific.
Oceania doesn't just include Australasia, but it also includes Polynesia, Melanesia, and Micronesia. It spans both the west and eastern hemispheres. Australasia is not just used geopolitically, but can also be used ecologically and physiogeographically. The term was coined in 1756 by Charles de Brosses and means south of Asia, which is derived from Latin.