Oxidase

The Oxidase referenced by the name is cytochrome C Oxidase, a protein on the electron transport chain important for oxidizing molecules. While not it's usual substrate, the reagent for this test (Tetramethyl-p-phenylenediamine) can also be oxidized. When oxidized this molecule turns a deep blue color from its usual colorless nature. The reagent can be added to a plate directly, or placed on an absorbent paper and chunks of organism smeared across it. either way a quick blue color will signal a positive result. A fresh plate is always the best for testing oxidase. This is partially why it's one of the first tests performed in this guide.