Constraints on the abundance of solid O-2 in dense clouds from ISO-SWS andground-based observations

The Short Wavelength Spectrometer (SWS) onboard the Infrared Space Observat ory (ISO) has been used to search for solid O-2 in cold dense clouds at 6.4 5 mu m. Additional constraints on the O-2 abundance are obtained from analy sis of the 4.67 mu m solid CO absorption profile observed from the ground. We derive upper limits of 50% and 100% of solid O-2 relative to solid CO to ward the protostellar sources R CrA IRS2 and NGC 7538 IRS9 respectively, co rresponding to abundances of 30 x 10(-6) and 15 x 10(-6) relative to n(H). These results indicate that the abundance of solid O-2 in dense clouds acco unts for less than 6% of the total oxygen budget in the interstellar medium . The reservoirs of oxygen in dense clouds are discussed, taking into accou nt recent measurements of oxygen-bearing species.