By means of a Zeeman modulation microwave spectrometer used with a lock-in amplifier, the 5 millimeter wave absorption lines of oxygen have been measured at low pressures, where they are completely resolved. Precise measurements of the frequencies of the lines have been made, and uniform deviations from the frequencies predicted by the theoretical formulas of Schlapp were found. The pressure variation of line-width has been measured for three of the observed lines, and found to be linear. The line-width parameter was found to vary with the rotational state. It is 0.053 ${\mathrm{cm}}^{\ensuremath{-}1}$/atmos. for $K=3$, and 0.021 ${\mathrm{cm}}^{\ensuremath{-}1}$/atmos. for $K=21$. Its order of magnitude is the same for low pressures (\ensuremath{\sim}${10}^{\ensuremath{-}1}$ mm Hg) as for high pressures (\ensuremath{\sim}1 atmos.).