We have investigated two techniques for making sub-Doppler frequency measurements with the CO overtone laser. We studied three OCS transitions whose frequencies overlap either directly with CO Av = 2 overtone transition frequencies or with the overtone lines after they have been shifted by an acousto-optic modulator. We have investigated both conventional saturated-absorption and an optical heterodyne polarization. While we eventually used the latter technique for our measurements, it is too cumbersome to use as a conventional laser stabilization tool. Saturation absorption is considerably simpler and has potential for a more accurate measurement, if some technical problems are overcome. This becomes more important in the light of a potential use of the CO Av=2 P 26 (9) transition in a new frequency chain. This transition was used for the OCS P(27) 10°l-00°0 measurement. Polarization spectroscopic techniques with optical heterodyne detection were used to observe the features and then to provide the discriminant for locking the overtone laser to the OCS transitions. A C0 2 laser synthesizer was used for the frequency measurement basis. The new frequencies (uncertainties in parentheses) resulting from the measurements are for the OCS 10°l-00°0 P(27), 87 117 278.492(50) MHz; OCS ll'l-OFO R(14), 87 222 001.143(70) MHz; and for the OC^S 10°1-00°0 P(9), 87 010 586.671(75) MHz.