A process for upgrading bio-oil comprising: a) combining a bio-oil comprising oxygenated hydrocarbons and having a total oxygen content greater than about 5 wt% with a diluent thereby forming a feed stream; b) charging the feed stream to a preheater and heating the feed stream to a temperature in the range of from about 80 to about 430°C in the preheater thereby forming a heated feed stream; c) charging the heated feed stream to a hydrodeoxygenation unit operated at a temperature in the range of from about 300 to about 500°C wherein at least a portion of the oxygen is removed from the oxygenated hydrocarbons of the bio-oil thereby forming an effluent stream comprising the diluent, water, and an at least partially upgraded bio-oil; and d) separating at least a portion of the at least partially upgraded bio-oil from the effluent stream thereby forming a product stream comprising the at least partially upgraded bio-oil having a total oxygen content lower than the total oxygen content of the bio-oil; wherein the diluent: (i) is present in the feed stream in an amount ranging from about 0.5 to about 50 wt%; and (ii) comprises a fatty acid methyl ester (FAME) and a portion of the at least partially upgraded bio-oil; wherein a portion of the oxygenated hydrocarbons in the feed stream react with each other in the preheater or the hydrodeoxygenation unit or both the preheater and the hydrodeoxygenation unit forming heavier compounds which are deposited in the preheater or the hydrodeoxygenation unit or both the preheater and the hydrodeoxygenation unit; and wherein the total amount of heavier compounds deposited in the preheater and in the hydrodeoxygenation unit is less than the total amount of heavier compounds deposited in the preheater and the hydrodeoxygenation unit when the feed stream does not contain the diluent; and wherein the temperature of the upgraded bio-oil is higher than the temperature of the bio-oil; and wherein the inclusion of the at least partially upgraded bio-oil in the feed stream provides heat to the feed stream thereby reducing the required heat duty of the preheater, reducing the heated surface temperatures of the preheater, and slowing the reaction of the oxygenated hydrocarbons in the feed stream with each other minimizing or preventing buildup of carbonaceous deposits in the preheater or the hydrodeoxygenation unit or both the preheater and the hydrodeoxygenation unit. a) combining a bio-oil comprising oxygenated hydrocarbons and having a total oxygen content greater than about 5 wt% with a diluent thereby forming a feed stream; b) charging the feed stream to a preheater and heating the feed stream to a temperature in the range of from about 80 to about 430°C in the preheater thereby forming a heated feed stream; c) charging the heated feed stream to a hydrodeoxygenation unit operated at a temperature in the range of from about 300 to about 500°C wherein at least a portion of the oxygen is removed from the oxygenated hydrocarbons of the bio-oil thereby forming an effluent stream comprising the diluent, water, and an at least partially upgraded bio-oil; and d) separating at least a portion of the at least partially upgraded bio-oil from the effluent stream thereby forming a product stream comprising the at least partially upgraded bio-oil having a total oxygen content lower than the total oxygen content of the bio-oil;