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:10464263, PubMed:31932717). May participate in synthesis of oncofetal fibronectin (PubMed:10464263). Has activity toward MUC1A, MUC2, EA2 and fibronectin peptides (PubMed:10464263). Glycosylates FGF23 (PubMed:31932717). . This is the function of ENSG00000139629 (GALNT6, polypeptide N-acetylgalactosaminyltransferase 6).