Once plants got reasonably established is why oxygen was able to build up in the atmosphere?
Plant cells resemble colonies of cyanobacteria and plants have mitochondria cells for respiration. This makes it more interesting since the first cyanobacteria and the first eukaryote without mitochondria originated at the same time.
Are you saying that cyanobacteria could not by itself produce the oxygen that we depend on today? Ocean plants that are eukaryotes with mitochondria and chloroplasts is why we have an o2 environment?