A potential valorization pathway for liquids obtained from lignocellulosic biomass could be their cohydrotreatment with petroleum cuts to produce transportation fuels. Thus, under hydrotreating conditions, biomass compounds are converted through HDO and decarbylation/decarboxylation reactions, leading to the production of CO, CO2, and water. Therefore, it seems essential to consider the impact of COx on the performance of an HDT catalyst in the conversion of a straight run gas oil (SRGO), working under industrial conditions. The conversion of the SRGO was performed at 330 °C, LHSV = 1 h−1, and 5 MPa on a commercial CoMo/Al2O3 sulfided catalyst in the presence of various concentrations of COx (0.0 < molar flow of CO or CO2 < 13.4 mmol/h). The introduction of COx inhibits the HDS and HDN reactions and, to a lesser extent, the hydrogenation of aromatics. Water gas shift and methanation reactions compete with HDT reactions, methane being the major product, and WGS equilibria govern the distribution of the remaining unconverted COx. The formed water is not responsible for the inhibition. From our experiments, it is clear that the inhibition previously observed in the coprocessing of esters or acids (propanoic acid and ethyldecanoate) can be attributed to the COx formed during the reaction.