1. As a gas in the atmosphere: This is the form we breathe. Oxygen makes up about 21% of the Earth's atmosphere.

2. Combined with other elements: This is called "bound oxygen" and is much more abundant than free oxygen. Here are some examples:

* Water (H2O): Oxygen makes up two-thirds of the mass of water.

* Rocks and minerals: Oxygen is the most abundant element in the Earth's crust, found in minerals like quartz (SiO2), feldspar, and calcite (CaCO3).

* Living organisms: Oxygen is a key component of carbohydrates, fats, proteins, and DNA.

* Other compounds: Oxygen is found in many other compounds, such as carbon dioxide (CO2), ozone (O3), and oxides of metals like iron oxide (rust).

How oxygen is formed:

* Photosynthesis: Plants and algae use sunlight to convert carbon dioxide and water into glucose and oxygen. This process releases oxygen into the atmosphere.

* Breakdown of water: Oxygen is also released when water is broken down by ultraviolet radiation in the upper atmosphere.

The importance of oxygen:

* Respiration: All living organisms, from plants to humans, use oxygen to breathe and convert food into energy.

* Combustion: Oxygen is essential for burning fuels like wood, coal, and natural gas.

* Oxidation: Oxygen is involved in many chemical reactions, including the rusting of iron and the decay of organic matter.

In short, oxygen is found all around us, both in the air we breathe and combined with other elements in the Earth's crust, water, and living organisms. It plays a crucial role in sustaining life and many chemical processes on our planet.