The effects of stabilizing fast pyrolysis oil (PO) with glycerol via catalytic glycerol pretreatment on upgrading via hydrodeoxygenation (HDO) or thermal treatment (TT) were studied. Nonstabilized (original) fast pyrolysis oil was also upgraded via HDO or TT to obtain benchmarks. Generally, HDO decreases the molecular weight of PO. The major beneficial effect of stabilization with glycerol was reduction in molecular weight of the upgraded oil. However, it was observed that the molecular weight reduction was largely induced by the dilution effect of glycerol/glycerol-derived fragments. It should also be noted that glycerol-consuming reactions via decreasing the carboxylic acid and phenolic contents of PO (e.g., via esterification) may play a role in reducing self-polymerization during HDO or TT. Stabilization of PO with glycerol, however, led to an increase in the yield of aqueous fractions of HDO and TT due to the formation of hydrophilic fragments from glycerol and PO constituents. After HDO, the oil fractions of stabilized and nonstabilized PO exhibited similar H/C and O/C molar ratios, suggesting that oxygen removal from the oil fractions was not significantly affected by stabilization.