Why was there no oxygen in the early atmosphere?

Answer 1

There was no ozone layer in the very beginning because the process of photosynthesis had to build it up over a long time.

The ozone layer built up starting over 2 billion years ago, and was dense enough (with enough ozone to protect biological organisms from lethal UV light) by about 600 million years ago. Prior to this period, life was restricted to the ocean.

Source: albany.edu/faculty/rgk/atm101/ozone.htm

Answer 2

There was no oxygen in the early atmosphere of the earth because there was no vegetation. When life finally began, vegetation provided the needed oxygen.

Answer 3

The land received too much harmful solar radiation