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. Has activity toward Muc5Ac and EA2 peptide substrates. This is the function of GALNT10 (polypeptide N-acetylgalactosaminyltransferase 10, Ensembl gene identifier ENSG00000164574).