Lanthipeptides represent a large class of cyclic natural products defined by the presence of lanthionine (Lan) and methyllanthionine (MeLan) cross-links. With the advances in DNA sequencing technologies and genome mining tools, new biosynthetic enzymes capable of installing unusual structural features are continuously being discovered. In this study, we investigated an O-methyltransferase that is a member of the most prominent auxiliary enzyme family associated with class I lanthipeptide biosynthetic gene clusters. Despite the prevalence of these enzymes, their function has not been established. Herein, we demonstrate that the O-methyltransferase OlvSA encoded in the olv gene cluster from Streptomyces olivaceus NRRL B-3009 catalyzes the rearrangement of a highly conserved aspartate residue to a β-amino acid, isoaspartate, in the lanthipeptide OlvA(BCSA). We elucidated the NMR solution structure of the GluC-digested peptide, OlvA(BCSA)GluC, which revealed a unique ring topology comprising four interlocking rings and positions the isoaspartate residue in a solvent exposed loop that is stabilized by a MeLan ring. Gas chromatography-mass spectrometry analysis further indicated that OlvA(BCSA) contains two dl-MeLan rings and two Lan rings with an unusual ll-stereochemistry. Lastly, in vitro reconstitution of OlvSA activity showed that it is a leader peptide-independent and S-adenosyl methionine-dependent O-methyltransferase that mediates the conversion of a highly conserved aspartate residue in a cyclic substrate into a succinimide, which is hydrolyzed to generate an Asp or isoAsp containing peptide. This overall transformation converts an α-amino acid into a β-amino acid in a ribosomally synthesized peptide, via an electrophilic intermediate that may be the intended product.