The electrochemical behavior of oxygen (O2) in blood was studied using boron-doped diamond (BDD) electrodes. Cyclic voltammogram of O2 in a 0.1 M phosphate buffer solution solution containing 1 × 10–6 M of bovine hemoglobin exhibits a reduction peak at −1.4 V (vs Ag/AgCl). Moreover, the scan rate dependence was investigated to study the reduction reaction mechanism, which was attributable to the reduction of O2 to H2O2 via two electrons. A linear calibration curve was observed in the concentration range of 86.88–314.63 mg L–1 (R2 = 0.99) with a detection limit of 1.0 mg L–1 (S/B = 3). The analytical performance was better than those with glassy carbon or platinum electrodes as the working electrode. In addition, an application to bovine blood was performed.