The protein quality control system in the endoplasmic reticulum (ER) ensures that only properly folded proteins are deployed throughout the cells. When nonnative proteins accumulate in the ER, the unfolded protein response is triggered to limit further accumulation of nonnative proteins and the ER is cleared of accumulated nonnative proteins by the ER-associated degradation (ERAD). In the yeast ER, aberrant nonnative proteins are mainly directed for the ERAD, but a distinct fraction of them instead receive O-mannosylation. In order to test whether O-mannosylation might also be a mechanism to process aberrant proteins in the ER, here we analyzed the effect of O-mannosylation on two kinds of model aberrant proteins, a series of N-glycosylation site mutants of prepro-alpha-factor and a pro-region-deleted derivative of Rhizopus niveus aspartic proteinase-I (Deltapro) both in vitro and in vivo. O-Mannosylation increases solubilities of the aberrant proteins and renders them less dependent on the ER chaperone, BiP, for being soluble. The release from ER chaperones allows the aberrant proteins to exit out of the ER for the normal secretory pathway transport. When the gene for Pmt2p, responsible for the O-mannosylation of these aberrant proteins, and that for the ERAD were simultaneously deleted, the cell exhibited enhanced unfolded protein response. O-Mannosylation may therefore function as a fail-safe mechanism for the ERAD by solubilizing the aberrant proteins that overflowed from the ERAD pathway and reducing the load for ER chaperones.