Explain how you know it must be oxygen. Where did the oxygen come from?

Hello, I didn’t super understand your question. But I’ll try my best to answer it for you. :)

At least half of Earth's oxygen comes from the ocean. Scientists estimate that 50-80% of the oxygen production on Earth comes from the ocean. The majority of this production is from oceanic plankton — drifting plants, algae, and some bacteria that can photosynthesize.

Like trees and other plants, phytoplankton can absorb sunlight and turn it into energy through a process called photosynthesis. Fortunately for us, photosynthesis also creates oxygen as a waste product, which the phytoplankton and plants expel — just like how humans and other animals produce carbon dioxide when we exhale.