The present invention relates to a method of ligating a first peptide via its C-terminus to the N-terminus of a second peptide, wherein the reaction is catalyzed by an asparagine/aspartate (Asx) peptide ligase OaAEPI Cys247Ala having the amino acid sequence of SEQ ID NO: 1. Further encompassed are a method of preparing a dimer, oligomer, or multimer of one or more peptides of interest and a method of modifying or tagging the surface of a target cell by one or more peptides of interest. Also encompassed in the invention are the ligated peptides and/or tagged target cells obtainable according to any of the methods, the peptide ligase OaAEPI Cys247Ala having the amino acid sequence of SEQ ID NO: 1, as well as kits comprising said peptide ligase.