Catalyzes the initial reaction in O-linked oligosaccharide biosynthesis, the transfer of an N-acetyl-D-galactosamine (GalNAc) residue from UDP-GalNAc to a serine or threonine residue on the protein receptor (PubMed:12407114, PubMed:22186971). Generates GalNAc-O-Ser/Thr structure also known as Tn antigen, which itself is immunogenic but also serves as a precursor for the synthesis of different mucin-type O- glycan core structures (PubMed:12407114). Contributes to the synthesis of O-linked glycans on mucins and proteoglycans of the central nervous system. May promote neurogenesis through glycosylation and stabilization of PDPN (By similarity) (PubMed:12407114, PubMed:22186971). {ECO:0000250|UniProtKB:Q8CF93, ECO:0000269|PubMed:12407114, ECO:0000269|PubMed:22186971}. [Isoform 1]: Can glycosylate both unmodified peptides and glycopeptides that already contain an O-linked GalNAc sugar. Transfers GalNAc to Thr-/Ser-rich tandem repeats GTTPSPVPTTSTTSAP of MUC5AC, specifically on Thr-3 of non-glycosylated MUC5AC peptide, on Thr-12 and Thr-13 of preglycosylated MUC5AC at Thr-3 (MUC5AC-3), on Thr-3 of preglycosylated MUC5AC at Thr-13 (MUC5AC-13) and on Thr-12 of preglycosylated MUC5AC at Thr-3 and Thr-13 (MUC5AC-3,13). Transfers GalNAc to three consecutive serine/threonine residues on SDC3 forming a triplet-Tn epitope expressed in Purkinje cells of the developing brain. . [Isoform 4]: Can glycosylate both unmodified peptides and glycopeptides that already contain an O-linked GalNAc sugar. Transfers GalNAc to Thr-/Ser-rich tandem repeats GTTPSPVPTTSTTSAP of MUC5AC, specifically on Thr-3 of non-glycosylated MUC5AC peptide, on Thr-12 and Thr-13 of preglycosylated MUC5AC at Thr-3 (MUC5AC-3), on Thr-3 of preglycosylated MUC5AC at Thr-13 (MUC5AC-13) and on Thr-12 of preglycosylated MUC5AC at Thr-3 and Thr-13 (MUC5AC-3,13). . This is the function of GALNT13 (polypeptide N-acetylgalactosaminyltransferase 13, ENSG00000144278).