ACE2 and COVID-19: OA was identified as a novel active constituent which blocked both the binding of SARS-CoV-2 Delta (B.1.617.2) and Omicron (B.1.1.529) RBD to ACE2 via residues ARG403, GLY496 and LYS353 in Delta RBD-ACE2 complexes and residues ARG403, HIS34 and LYS353 in Omicron RBD-ACE2 complexes, suggesting the therapeutic candidate of OA for COVID-19 treatment.