An oak bio-oil was aged at 90 degrees C using various times and methods. A novel method for aging bio-oils under shear is introduced and compared to standard (quiescent) aging experiments. In a hermetically sealed concentric cylinder rheometer, aging with shear for 8, 16, and 24 h showed increases in viscosity of 57, 300, and 720%, respectively. A similar increase in viscosity was observed after quiescently aging of sealed samples in a forced air oven (100, 120, and 740% after 8, 16, and 24 h, respectively). Another aging experiment under shear consisted of three 8 h aging steps with intermediate viscosity measurements. Viscosity increases were comparable to the 8, 16, and 24 h tests. A control experiment in the rheometer without shear found the increase in viscosity to be 30-50% less than the sheared experiments. The number-average molecular weight increased as samples were heat-treated at 90 degrees C for longer times. The water content showed small increases and decreases with aging, which was attributed to the heterogeneity of the sample. Real-time viscosity measurements during the 90 degrees C aging step found that the rate of viscosity growth decreased over time. An exponential decay function estimated the viscosity to be 90% of the steady-state viscosity after similar to 3 days at 90 degrees C.