We present initial results from the Herschel GT key program: Herschel observations of EXtra-Ordinary Sources (HEXOS) and outline the promise
and potential of spectral surveys with Herschel/HIFI. The HIFI instrument offers unprecedented sensitivity, as well as continuous spectral coverage
across the gaps imposed by the atmosphere, opening up a largely unexplored wavelength regime to high-resolution spectroscopy. We show the
spectrum of Orion KL between 480 and 560 GHz and from 1.06 to 1.115 THz. From these data, we confirm that HIFI separately measures the dust
continuum and spectrally resolves emission lines in Orion KL. Based on this capability we demonstrate that the line contribution to the broad-band
continuum in this molecule-rich source is ~20−40% below 1 THz and declines to a few percent at higher frequencies. We also tentatively identify
multiple transitions of HD^(18)O in the spectra. The first detection of this rare isotopologue in the interstellar medium suggests that HDO emission is
optically thick in the Orion hot core with HDO/H_2O ~ 0.02. We discuss the implications of this detection for the water D/H ratio in hot cores