Fiber-optic reflectance measurements were performed on scattering phantoms containing red blood cells. The oxygen pressure in the phantom was decreased by nitrogen gas bubbling, while carefully controlling and monitoring the temperature $(37°\mathrm{C})$ and pH (7.4). The blood oxygen saturation $\left(\mathrm{S}{\mathrm{O}}_2\right)$ was extracted from the optical spectroscopic measurements by fitting the spectra over the wavelength range of $350–850\mathrm{nm}$ to a model that includes the effects of pigment packaging. The effect of using different oxy- and deoxyhemoglobin extinction spectra on the quality and accuracy of the fits is analyzed. The optically extracted $\mathrm{S}{\mathrm{O}}_2$ is compared with the standard oxygen dissociation curve (ODC). Depending on the choice of hemoglobin extinction spectra the absolute deviation between the ODC and the optically extracted $\mathrm{S}{\mathrm{O}}_2$ is less than 2.5% over the measured range of saturations (4%–99%).