Catalyzes the transfer of a single N-acetylglucosamine from UDP-GlcNAc to a serine or threonine residue in extracellular proteins resulting in their modification with a beta-linked N-acetylglucosamine (O-GlcNAc). Specifically glycosylates the Thr residue located between the fifth and sixth conserved cysteines of folded EGF-like domains. 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 Widely expressed. Expressed in brain, heart, kidney, lung, skeletal muscles and thymus. Highest expression is observed in lung and the lowest in skeletal muscles. Expressed at embryonic day (E) 10.5 in the growing edge of the limb buds. At 11.5 dpc, enriched in the apical ectodermal ridge of the limbs. By 12.5 dpc, expression assumes a digit-condensation pattern in the 4 limbs. Belongs to the glycosyltransferase 61 family.