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. Involved in epithelial cell adhesion/interaction with the extracellular matrix by mediating glycosylation of proteins in the secretory pathway, such as Dumpy (Dp). 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 Expressed both maternally and zygotically. Highly expressed in preblastoderm-stage embryos. During the later stages of embryogenesis, expression is ubiquitous with the level progressively decreasing. Defects in apical extracellular matrix. Embryos show defects in the formation of the innermost layer of the apical extracellular matrix and its attachment to the epidermis. Most larvae die during second-instar or second/third-instar interface, but some survive until early third-instar. Surviving larvae display cuticle defect and irregular tracheal morphology. Ultrastructural analysis of larval epidermis reveals disruption of the deposition zone of the endocuticle, leading to separation of the epidermis from the chitin layers.