So a very good preprint has come out that had a bunch of experiments and results about differences between Delta and Omicron:

Delta vrs Omicron in Mice and Hamsters
You can watch TWIV 851 on YouTube if you are interested in a discussion.

A whole bunch of different labs, in different countries, infected various mouse and hamster strains with both Delta and Omicron variants. Important findings: The degrees of pathology in their upper respiratory tracts was essentially the same between variants, but Delta deep lung pathology was more severe than Omicron. More severe including death in some cases. Not that Omicron did not cause deaths in certain sets of animals, but Delta caused significantly more. These differences were measured in several different ways, by different labs, and for different animals.

BUT (and it's a pretty big but) some of the most severe pathology was in a transgenic hamster strain that made human ACE2 receptors. And mouse and human immune systems are pretty different. So their conclusions are just about mice and hamsters, not humans. They are doing more work to track down why Omicron is less pathological for mice and hamsters. If we share those same mechanisms with mice and hamsters, then they might be able to say something about the relative pathogenicity of Omicron versus Delta in humans.

This is an excellent illustration of why real research is difficult: They can't do a lot of these experiments in humans. And when they do them in a (necessarily) imperfect animal model, they can't immediately draw conclusions about humans. But if they track down a particular RNA mutation that causes a particular protein change, where we share that affected biochemical activity with the animal model, then they can say how it could affect humans. It's all about determining mechanisms.

Another paper came out recently that said the changes to the spike RNA have nothing to do with virulence. They only affect spike binding and antibody evasion, and thus entry into the cell. Other mutations affect the proteins the virus makes once it's in the cell and has hijacked the replication mechanisms. Those are the culprits. But this may be more information than most of us need!

Educating anyone benefits everyone.