Marine fuel oil compositions are provided that exhibit unexpectedly high cetane numbers after addition of a cetane improver. Methods of making such compositions are also provided. The unexpected nature of the marine fuel oil compositions is based in part on the ability to achieve a substantial improvement in estimated cetane number by addition of a cetane improver to a hydrocarbonaceous composition with a natural estimated cetane number of less than 35. These unexpectedly high increases in estimated cetane number for fuels or fuel blending components with low natural estimated cetane numbers can allow for production of fuel compositions with desirable combustion characteristics while also maintaining a higher level of aromatic compounds and/or reducing or minimizing the amount of distillate boiling range components in the fuel or fuel blending component.