Spectroscopy of the nu(1) band of the astrophysically relevant ion HCO+ is performed with an optical parametric oscillator calibrated with an optical frequency comb. The sub-MHz accuracy of this technique was confirmed by performing a combination differences analysis with the acquired rovibrational data and comparing the results to known ground state rotational transitions. A similar combination differences analysis was performed from the same data set to calculate the previously unobserved rotational spectrum of the nu(1) vibrationally excited state with precision sufficient for astronomical detection. Initial results of cavity enhanced sub-Doppler spectroscopy are also presented and hold promise for further improving the accuracy and precision in the near future.