A frequency doubled erbium doped modelocked fiber frequency comb is used to implement a THz photomixing synthesizer. The useful THz linewidth is in order of 150 kHz and has been assessed along with the frequency accuracy by spectroscopic measurements demonstrating a relative accuracy of 10(-8) at frequencies around 1 THz. The THz synthesizer is used to implement a THz spectrometer to study the rotational absorption spectrum of carbonyl sulfide (OCS). Measurement of the principal transitions between 813 GHz and 1283 GHz allowed the properties of the THz spectrometer to be compared with competing techniques, and demonstrates the potential of the THz photomixing synthesizer as an alternative means to explore the THz domain.