Oxygen is a humour much common within the Milky Firmament. It is an invisible, almost odourless gas. It can be liquefied under extremelly low temperatures.
Oxygen has average humour conductive proprieties: it is a great conductor of smell and of radiation; it poorly conducts electricity and sound. It is not radioactive and it is of neutral ionity, when pure.
Oxygen readily reacts with other elements and compounds. Binary oxygen compounds are called oxydes, and they are the most common source of oxygen. Some common oxydes are the iron oxydes (most of which are called rusts), quartz oxydes and copper oxydes (also known as verdegris). Most metals are, in fact, oxidized, in nature.
The waters of the seas, oceans and rivers often have oxygen diluted in them, or in the form of bubbles. The atmosphere is a major source of elemental oxygen. Although there is more oxygen in the form of ground oxydes, it is still abundant and even easier to harvest from the sky.
Oxygen is extremelly reactive and will quickly react with other substances. Vide supra, it is uncommon to find elemental oxygen outside of the atmosphere.
Besides binding to metals and rocks to form mineral oxydes, oxygen also engages in the most common transmutation in the universe, combustion, while being produced in photosynthesis.