Chip-scale molecular clocks (CSMCs) perform frequency stabilization by referencing to the rotational spectra of polar gaseous molecules. With potentially the "atomic" clock grade stability, cm 3 -level volume and <100 mW DC power, CSMCs are highly-attractive for the synchronization of high speed radio access network (RAN), precise positioning and distributed array sensing. However, the medium/long-term stability of CSMCs is hindered by the transmission baseline tilting due to the uneven frequency response of the spectroscopic system and the molecular cell. To enhance the medium/long-term stability, this paper presents a CSMC architecture locking to the high-oddorder dispersion curve of the 231.061 GHz rotational spectral line of carbonyl sulfide (OCS) molecules, which is selected as the clock reference. A monolithic THz transceiver generates a high-precision, wavelength-modulated probing signal. Then, the wave-molecule interaction inside the molecular cell translates the frequency error between the probing signal and the spectral line center to the periodic intensity fluctuation. Finally, the CSMC locks to the 3 rd -order dispersion curve after a phase-sensitive lock-in detection. In addition, a pair of slot array couplers is employed as an effective chip-to-molecular cell interface. It leads to not only a higher SNR, but also a significantly simplified CSMC package. Implemented on a 65 nm CMOS process, the high-order CSMC presents a measured Allan deviation of 4.3×10 −11 under an averaging time of τ =10 3 s, while consuming 70.4 mW DC power.