We report the measurement of the hemoglobin (Hb) oxygen saturation level in human blood with a spectroscopic spectral-domain optical coherence tomography (SSD-OCT) system based on the crossover behavior of Hb and oxyhemoglobin $\left(\mathrm{Hb}{\mathrm{O}}_2\right)$ absorption coefficients around $800\mathrm{nm}$ . By calculating the ratio of the exponential decay constant of A-mode scan signal in the long-wavelength range $(>800\mathrm{nm})$ over that in the short-wavelength range $(<800\mathrm{nm})$ , the relative oxygen saturation level of Hb can be calibrated. Such a relative level can be used for practical diagnosis application after a golden standard is built by comparing the variation of the OCT result with that of a conventional method, such as the use of a blood gas analyzer. In our experiment, the variation curve based on the SSD-OCT measurement, which has the advantages of noninvasive, real-time, and high-resolution measurements, is coincident with that of using a commercial blood gas analyzer.