UniProt Summary: 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. Does not glycosylate apomucin or SDC3. {ECO:0000269|PubMed:10978536, ECO:0000269|PubMed:12407114}.