Catalyzes the initial reaction in O-linked oligosaccharide biosynthesis, the transfer of an N-acetyl-D-galactosamine residue to a serine or threonine residue on the protein receptor (PubMed:8748160). Has a broad spectrum of substrates such as apomucin-, MUC5AC-, MUC1- and MUC2-derived peptides (By similarity). L-seryl-[protein] + UDP-N-acetyl-alpha-D-galactosamine = 3-O-[N-acetyl-alpha-D-galactosaminyl]-L-seryl-[protein] + H(+) + UDP L-threonyl-[protein] + UDP-N-acetyl-alpha-D-galactosamine = 3-O-[N-acetyl-alpha-D-galactosaminyl]-L-threonyl-[protein] + H(+) + UDP Protein modification; protein glycosylation. There are two conserved domains in the glycosyltransferase region: the N-terminal domain (domain A, also called GT1 motif), which is probably involved in manganese coordination and substrate binding and the C-terminal domain (domain B, also called Gal/GalNAc-T motif), which is probably involved in catalytic reaction and UDP-Gal binding. The ricin B-type lectin domain directs the glycopeptide specificity. It is required in the glycopeptide specificity of enzyme activity but not for activity with naked peptide substrates, suggesting that it triggers the catalytic domain to act on GalNAc-glycopeptide substrates (By similarity). Belongs to the glycosyltransferase 2 family. GalNAc-T subfamily.