Addition of nucleotide-activated sugars directly onto the polypeptide through O-glycosidic linkage with the hydroxyl of serine or threonine. L-seryl-[protein] + UDP-N-acetyl-alpha-D-glucosamine = 3-O-(N-acetyl-beta-D-glucosaminyl)-L-seryl-[protein] + H(+) + UDP L-threonyl-[protein] + UDP-N-acetyl-alpha-D-glucosamine = 3-O-(N-acetyl-beta-D-glucosaminyl)-L-threonyl-[protein] + H(+) + UDP Protein modification; protein glycosylation. The TPR repeat domain mediates recognition of protein substrates. Cells lack O-GlcNAc post-translational modification and brood size decreases with increasing ambient temperature. Belongs to the glycosyltransferase 41 family. O-GlcNAc transferase subfamily.