It appears to be spreading twice as fast as Delta in the Gauteng province; that may mean it is more transmissible than Delta, or only that it is better at evading preexisting immunity.  There is evidence that Omicron can reinfect those who had earlier variants at a rate three times higher than previous variants did.   Less than one third of the population is vaccinated in South Africa, but there have been several previous waves of infection there, and one imagines those previously infected take fewer precautions.  So it is possible–but this is only speculation at this stage–that Omicron is outperforming Delta because it is infecting those who had recovered from earlier variants.   The big question, still, is how well vaccination protects against the Omicron variant.  I have not yet seen any data on breakthrough infections of Omicron; feel free to add relevant links in the comments.