Omicron’s ancestors may have spent a significant amount of time adapting to mouse cells, before re-entering human hosts. Omicron appears selected to replicate primarily in the bronchial tract. Deeper in the human lung, it functions far less efficiently than Delta or the first strains from Wuhan. This is probably why it causes mostly mild illness, and it is reminiscent of techniques used to make live attenuated influenza vaccines safer for use in humans. Such vaccines are cold-adapted, that is, selected to circulate primarily in the cooler upper respiratory tract rather than in the warmer, more vulnerable lungs.
The balance of the evidence is that Omicron leaked from a lab engaged in SARS-2 vaccine research.