Which of the following gases is not a greenhouse gas?

The greenhouse effect is the heating of the earth and its surrounding caused by solar energy trapped by carbon dioxide and other gases in the atmosphere.

The name greenhouse comes from the glass structure known as a greenhouse, which is used in colder climates to produce green vegetation.

Greenhouse gases include carbon dioxide, methane, water vapour, nitrous oxide, chlorofluorocarbons, and ozone.

The oxygen gas is not regarded as a greenhouse gas.

Hence, the correct option is A, i.e. Oxygen.