The tumor associated Tn (GalNAcalpha(1-1)-Thr/Ser)- and T (Galbeta(1-3)-GalNAcalpha(1-1)Thr/Ser)-antigens and their sialylated derivatives are present on the surface of many cancer cells. Preparative synthesis of these sialylated T- and Tn-structures has been achieved mainly from a chemical synthetic approach due to the lack of the required glycosyltransferases. We demonstrate a flexible and efficient chemoenzymatic approach for using recombinant sialyltransferases including a chicken GalNAcalpha2,6-sialyltransferase (chST6GalNAc I) and a porcine Galbeta(1-3)GalNAcalpha-2,3-sialyltransferase (pST3Gal I). Using these enzymes, the common O-linked sialosides Neu5Acalpha(2-6)GalNAcalpha(1-1)Thr, Galbeta(1-3)[Neu5Acalpha(2-6)]GalNAcalpha(1-1)Thr, Neu5Acalpha(2-3)Galbeta(1-3)GalNAcalpha(1-1)Thr, and Neu5Acalpha(2-3)Galbeta(1-3)[Neu5Acalpha(2-6)]GalNAcalpha(1-1)Thr were readily prepared at preparative scale. The chST6GalNAc I was found to require at least one amino acid (Thr/Ser) for optimal activity, and is thus an ideal catalyst for synthesis of synthetic glycopeptides and glycoconjugates with O-linked glycans.