Even though electrically non-polar, oxygen gas absorbs microwaves because the magnetic moment of the ${\mathrm{O}}_{2}$ molecule interacts with electromagnetic fields. The resulting absorption is most pronounced, exceeding 10 db/km, for wave-lengths in the vicinity of \textonehalf{} cm, for then there is resonance to the spacings in the "rho-type triplet" or spin fine-structure in the $^{3}\ensuremath{\Sigma}$ ground state of ${\mathrm{O}}_{2}$. There is also a subsidiary resonance near \textonequarter{} cm, and a non-resonant absorption at long wave-lengths due to diagonal matrix elements. The calculated values of the absorption are given in Table II and depend on the choice of the line-breadth constant $\ensuremath{\Delta}\ensuremath{\nu}$ which represents the effect of broadening by collision. Comparison is made of these theoretical results with the absorption in the \textonehalf{} cm region, observed by various experimentalists with different methods. It is concluded that 0.02 ${\mathrm{cm}}^{\ensuremath{-}1}$ is probably the best choice for $\frac{\ensuremath{\Delta}\ensuremath{\nu}}{c}$. The theoretical dependence of the absorption on pressure is discussed, and is particularly interesting because of the relation to the mechanism of collision-broadening and because the resonances to individual rotational lines are resolved at low pressures.