Huh? There's O2 on comets ?! More to the point, without plants to renew it, the oxygen will quickly react and disappear.
Big time oxygen production didn't begin until plants colonized land.
Demonstrably false. I will give two reasons, there are in fact more:
1) The so-called Cambrian explosion. The arthropods, worms, chordates, etc found in the Burgess shale and suchlike depend on oxygen, so it had to be present before the "explosion". There were no land plants at that time.
It is believed that land plants evolved directly from a freshwater (versus a marine or salt-water) green algal ancestor sometime in the mid-Ordovician, between 450 to 470 million years ago (MYA) (Graham, 1993, p 234-235). The earliest known seedless vascular plant (genus: Cooksonia) existed in the mid-Silurian, probably around 420 MYA and eventually went exinct by the early Devonian (about 390 MYA) (ref. 3-4).source
2) Banded iron formations. These are deposits of iron ore that occurred when the atmospheric oxygen reached a particular level. They are from 1.8 to 2.5 billion years old, probably before there were any eukaryotes.
He's full of it, of course, as you point out. In addition, 60-70% of the primary productivity today comes from the ocean and certainly that was closer to 100% before land plants. Primary productivity is a direct measurement of oxygen production. Stromatolites go way back and they are partly the products of blue green algae - i.e. oceanic oxygen production. BGA are still responsible for close to half of the primary productivity even today.