What resulted when the early atmosphere saw an increase in oxygen?

after the Cambrian explosion, photosynthesis slowly built up the oxygen content in the atmosphere. As this oxygen content increased, minerals that could react with oxygen did react with oxygen as they were exposed to the air.

So during the Devonian period (about 400 m.y.a.) any iron present was oxidised into the Fe3+ form, resulting in sedimentary rocks that contain a strong red characteristic colour (red Devonian sandstone).Britannica - old red sandstone

Since the majority of organisms at the time were obligate anaerobes (obligated to exist anaerobically or without oxygen), the increase in oxygen resulted in what is arguably the largest mass extinction event on the Earth.

The increase in oxygen in the early atmosphere resulted in the Great Oxygenation Event, which occurred around 2.4 billion years ago. This event led to the oxygenation of Earth's atmosphere and oceans, paving the way for the development of aerobic organisms and significantly altering the planet's environment.