Spectral indices (delta m2100 and delta m1500) for 32 O-type stars with known spectral class and reddening are plotted versus log of effective temperature. The delta m2100 indices are considered to be almost luminosity-independent, while evidence of a well-established relationship between observed spectral indices and empirical temperatures appears. Also graphed are equivalent widths of selected photospheric lines versus temperature. Theoretical line strengths computed with a code that solves the LTE line transfer problem from the model atmosphere input of Kurucz, et al. (1974) indicate that the general behavior of the silicon lines, but not the P V resonance line, appears consistent with the computations.