TEO-GalNAc is a chemoenzymatic strategy for tag-free enrichment and quantitative mapping of Tn/T O-GalNAc glycopeptides. Ketone-modified CMP-Sia is transferred by ST6GalNAc1 for selective tagging, followed by reversible oxime capture using hydroxylamine beads. Mild acid release enables tag-free recovery and quantitative assessment of sialylation occupancy on core GalNAc residues in complex samples.