O-Linked β-N-acetylglucosamine (O-GlcNAc) is a monosaccharide that plays an essential role in cellular signaling throughout the nucleocytoplasmic proteome of eukaryotic cells. Strategies for selectively increasing O-GlcNAc levels on a target protein in cells would accelerate studies of this essential modification. Here, we report a generalizable strategy for introducing O-GlcNAc into selected target proteins in cells using a nanobody as a proximity-directing agent fused to O-GlcNAc transferase (OGT). Fusion of a nanobody that recognizes GFP (nGFP) or a nanobody that recognizes the four-amino acid sequence EPEA (nEPEA) to OGT yielded nanobody-OGT constructs that selectively delivered O-GlcNAc to a series of tagged target proteins (e.g., JunB, cJun, and Nup62). Truncation of the tetratricopeptide repeat domain as in OGT(4) increased selectivity for the target protein through the nanobody by reducing global elevation of O-GlcNAc levels in the cell. Quantitative chemical proteomics confirmed the increase in O-GlcNAc to the target protein by nanobody-OGT(4). Glycoproteomics revealed that nanobody-OGT(4) or full-length OGT produced a similar glycosite profile on the target protein JunB and Nup62. Finally, we demonstrate the ability to selectively target endogenous α-synuclein for O-GlcNAcylation in HEK293T cells. These first proximity-directed OGT constructs provide a flexible strategy for targeting additional proteins and a template for further engineering of OGT and the O-GlcNAc proteome in the future. The use of a nanobody to redirect OGT substrate selection for glycosylation of desired proteins in cells may further constitute a generalizable strategy for controlling a broader array of post-translational modifications in cells.