  We present absolute frequency measurement of the unperturbed P7 P7 O$_2$
B-band transition with relative standard uncertainty of $2\times10^{-11}$. We
reached the level of accuracy typical for Doppler-free techniques, with
Doppler-limited spectroscopy. The Doppler-limited shapes of the P7 P7 spectral
line were measured with a frequency-stabilized cavity ring-down spectrometer
referenced to an $^{88}$Sr optical atomic clock by an optical frequency comb.
