Where did this oxygen come from? Plants?

Earth probably only had high oxygen in its second half of existance.
There are early sedimentary minerals impossible at high oxygen.
The first half was probably mostly CO2 atmosophere from primordial
outgassing. Venus and Mars are currently mostly CO2.

Oxygen was a poison to early earth life. The simplest life is anaerobic.
Only later did life develop additional metabolic systems for excreting
surplus oxygen, or burning it for energy. These are not perfect. Oxygen
posioning is thought to contribute to diseases of aging and death in
higher organisms.