This paper describes spectral and intensity calibration of a Spatial Heterodyne Spectrometer (SHS) designed to observe radial velocity resolved profiles of diffuse [OII] 3726 Å and 3729 Å emission lines from the warm (104 K), low-density (10-1 cm-3), ionized component of our Galaxy's interstellar medium. Measurements were carried out using calibration lamp and sky data in order to assess the [OII] SHS's sensitivity as a function of input aperture, a necessary step in the analysis of planetary nebula (PNe) datasets to be used in the instrument’s final intensity calibration. Fully calibrated [OII] line intensities will help quantify variations in temperature and in ionization state within the WIM.