Compared to the Delta variant, the Omicron has a higher mutation in their Spike Proteins, meaning they may have a greater binding capacity to ACE2 receptors present in the alveoli cells, leading to an increased entry of viruses into your lungs. Genetic mutations mean the Omicron may have a higher replication rate and viral load than the Delta variant.
Further, higher mutations in the S-protein could mean a lower antigen-antibody immune response generated by the vaccine, i.e., which may cause a decreased efficacy of the vaccine.
Also, the Reproductive Rate (degree of spreadability of the infection, i.e., how many people can one person infect) of Omicron is significantly higher than in the Delta. i.e., one infected person can infect up to six other people. These figures are very evident, considering the recent events in South Africa. The Omicron variant has only taken a few weeks to spread compared to the Delta variant, which took a couple of months.