We are exposed to endogenous reactive oxygen species (ROS) produced during normal aerobic metabolism and by the innate immune systems. Excessive production of ROS is critically important in disease onset and progression. Post-translational oxidative modifications of hemoglobin have been used as a surrogate biomarker for monitoring the oxidative stress in vivo. In this study, the Fenton reaction was used as a model to produce ROS and react with human hemoglobin. After trypsin digestion, the types and sites of modifications were characterized by a nanoflow LC-nanoelectrospray ionization high-resolution mass spectrometer. Besides oxidation at certain sites of Met, His, and Tyr, conversion of histidine to aspartate and hydroxyaspartate was identified at four sites. Furthermore, advanced oxidation of histidine to hydroxyaspartate was identified at two sites. Elevated oxidative stress is tightly associated with oral cancer. The relative extent of modification at the identified sites was quantified relative to the native peptide present in the digest as the reference peptide in hemoglobin from 18 oral cancer patients and 15 healthy control subjects. The results revealed that the extents of oxidation at β-His-77 and β-Asp-99 of globin were significantly elevated in oral cancer patients compared to healthy subjects, while the extents of conversion of histidine residues at α-His-20, α-His-50, and β-His-2 to aspartate were significantly decreased. Furthermore, the advanced oxidation of histidine to hydroxyaspartate at α-His-50 and β-His-2 is also significantly higher in oral cancer patients than in healthy subjects (p < 0.05). To our knowledge, this advanced oxidation of histidine to hydroxyaspartate is a new post-translational protein modification, and it is found in human hemoglobin isolated from blood. This advanced oxidative modification in hemoglobin might be a potential biomarker to assess oxidative stress in oral cancer patients.