Catalysis of the reaction: UDP-N-acetyl-D-glucosamine + -L-serine = UDP + -3-O-(N-acetyl-D-glucosaminyl)-L-serine, or UDP-N-acetyl-D-glucosamine + -L-threonine = UDP + -3-O-(N-acetyl-D-glucosaminyl)-L-threonine.
protein O-GlcNAc transferase activity