Asparagine-linked glycosylation of proteins is an essential cotranslational and posttranslational protein modification in plants. The central step in this process is the transfer of a preassembled oligosaccharide to nascent proteins in the endoplasmic reticulum by the oligosaccharyltransferase (OST) complex. Despite the importance of the catalyzed reaction, the composition and the function of individual OST subunits are still ill defined in plants. Here, we report the function of the highly conserved OST subunit OST3/6. We have identified a mutant in the OST3/6 gene that causes overall underglycosylation of proteins and affects the biogenesis of the receptor kinase EF-TU RECEPTOR involved in innate immunity and the endo-β-1,4-glucanase KORRIGAN1 required for cellulose biosynthesis. Notably, the ost3/6 mutation does not affect mutant variants of the receptor kinase BRASSINOSTEROID-INSENSITIVE1. OST3/6 deficiency results in activation of the unfolded protein response and causes hypersensitivity to salt/osmotic stress and to the glycosylation inhibitor tunicamycin. Consistent with its role in protein glycosylation, OST3/6 resides in the endoplasmic reticulum and interacts with other subunits of the OST complex. Together, our findings reveal the importance of Arabidopsis (Arabidopsis thaliana) OST3/6 for the efficient glycosylation of specific glycoproteins involved in different physiological processes and shed light on the composition and function of the plant OST complex.