Discoveries of the microwave spectra of a considerable number of molecules prove the existence of rather complex molecules in the interstellar clouds, and provides the possibility to study dense clouds, which are obscured by the dust they contain. The HO molecule has been observed in the interstellar medium since 1963 through its 18-cm transitions. In the subsequent years HO was observed by its rotational transitions at far infrared wavelengths, mainly in the Orion region.  Because each rotational level of HO is split in by lambda doubling, astronomers can observe a wide variety of energy states from the ground state.