How did the atmosphere get its oxygen

Because super tiny organisms called cyanobactierea (alge) they conducted photosynthesis and they us sunshine water and Carbon D to make Oxygen

Tiny organisms know as cyanbacteria, or blue-green algae. These microbes conduct photosynthesis : using sunshine, water and carbon dioxide to produce carbohydrates and, yes, oxygen.

There's your answer. Hope This Helps :)