===================================================================== ==> In this proposal, more time is being requested for TPHILLIP.TGP_OI ==> This proposal requests an upgrade from Priority 3 for TPHILLIP.TGP_OI ===================================================================== We propose to use ISO observations of the 63 micron line of OI to determine the gas phase oxygen abundance in dense interstellar clouds, where complex molecules are found and where the interstellar UV radiation field is attenuated by dust. It has been shown that carbon can be largely accounted for by CO and CI measurements, but the predicted molecular reservoirs of oxygen, O2 and H2O, have been shown not to account for the anticipated gas phase oxygen by at least an order of magnitude. We expect that the remainder is largely atomic oxygen and propose a method of 63 micron absorption spectroscopy to identify the missing oxygen content of these clouds. In order to separate OI emission from the background continuum source from OI absorption by the foreground source we select lines of sight where the velocity separation of emitter and absorber is greater than the LWS Fabry Perot resolution of 30 km/s. Also, to avoid the effects of strong PDRs we choose absorbing clouds well removed from ionizing stars. Further, to aid in the understanding of the relation between the oxygen in the PDRs at the cloud surfaces, generated by the ambient interstellar UV field, and oxygen associated with the more shielded molecular region, we measure a set of clouds with a range of H2 column densities. The cloud OI absorption should correlate with the H2 column densities (determined from trace molecules), whereas the PDR contribution should be roughly constant. We had originally proposed to measure 9 sources, but due to the reduced sensitivity of LWS and increased observing overhead (mostly due to the necessity of removing fringes) only 4 will be completed in priority one time already allocated. This new proposal will permit the original goal to be achieved.