Final answer:
The oxygen in the air comes mainly from plants and microorganisms through photosynthesis, a process that transforms carbon dioxide and water into glucose while releasing oxygen.
Step-by-step explanation:
The oxygen in the air comes primarily from plants through a process called photosynthesis. During this process, plants use sunlight to convert carbon dioxide and water into glucose and release oxygen as a byproduct. Not only do plants on land contribute to this, but also microorganisms such as cyanobacteria and phytoplankton in the oceans play a significant role in oxygen production. This interplay of biological processes underlines the importance of plant and microbial life in maintaining the Earth's atmosphere suitable for animals, including humans, to breathe.