Catalysis of the reaction: UDP-N-acetyl-D-glucosamine + RNA polymerase II large subunit CTD heptapeptide repeat (YSPTSPS) = UDP + RNA polymerase II large subunit CTD heptapeptide repeat 3-O-(N-acetyl-D-glucosaminyl)-L-serine (position 7).
RNA polymerase II C-terminal domain S7 O-GlcNAc transferase activity