Comprehensive Summary O‐Mannosylation plays a vital role in the regulation of a variety range of biological processes, for instance, brain and muscle development. However, the precise function remains largely unknown due to its innate heterogeneity. In this regard, it is still welcome to develop efficient methods to access diverse structurally‐defined glycopeptides. In this study, a diversity‐oriented assembly of O‐mannosyl α‐dystroglycan (α‐DG) glycopeptides has been achieved via a chemoenzymatic strategy. This strategy features (i) gram scale divergent synthesis of core m1, core m2 and core m3 mannosylated amino acids from judiciously designed protecting group strategies and chemical glycosidation; (ii) efficient glycopeptide assembly via the optimized microwave‐assisted solid phase peptide synthesis (SPPS); and (iii) enzymatic elaboration of the core glycan structures to install galactosyl and sialyl‐galactosyl moieties. The efficiency and flexibility of this chemoenzymatic approach was demonstrated with the construction of 12 glycopeptides with different core m1, core m2 and core m3 mannosyl glycans, including a core m2 glycopeptide bearing a heptasaccharide for the first time.