On 25 November, South Africa announced that it was tracking a new variant, B.1.1.529/Omicron, and shared the following:

New variant detected in South Africa (lineage B.1.1.529) with high number of mutations, which are concerning for predicted immune evasion and transmissibility

B.1.1.529 genomes produced from samples collected 12-20 Nov from Gauteng, SA (n=77), Botswana (n=4) and Hong Kong (n=1, traveler from SA)

B.1.1.529 can be detected by one particular PCR assay (before whole genome sequencing)

Early signs from diagnostic laboratories that B.1.1.529 has rapidly increased in Gauteng and may already be present in most provinces

Mutation profile predicted to give significant immune evasion and enhanced transmissibility

See also these three assessments by the UK HSA, Belgian NRL, and WHO respectively.

Will the Omicron variant be less lethal than Delta?

This will resolve positive if, out of the first 4 peer-reviewed published studies that provide estimates of the odds/risk/hazard of death due to infection with the Omicron variant, at least 3 indicate that the odds/risk/hazard of death due to infection with the Omicron is less than that due to infection with the Delta variant.

The relevant studies must be rigorous and must make direct comparisons between infections with Omicron in a group and infections with Delta in a similar group. Moreover, at the very least, the variables of age and comorbidities should be controlled for.

At least 3 of the first 4 relevant studies that meet the above criteria should specify that they've found a statistically significant decrease (p < 0.05) in Omicron's lethality relative to Delta. Otherwise, this resolves negatively —e.g. negative resolution will occur if only 2 clearly state they've found a statistically significance decrease, 1 has mixed results, and 1 says that they did not find a statistically significant decrease.

Also see this question on whether the Omicron variant will be more lethal than Delta.