The abundance of molecular oxygen in dense molecular clouds is a key
 problem within astrochemistry. Because of its great reactivity (e.g. burning),
 the availability of oxygen regulates the abundances of almost all molecules.
 The interstellar medium is thought to be oxygen-rich (elemental C/O=0.5) and
 all theoretical models predict that the oxygen not locked up in CO will be in
 O2. However, despite sensitive searches, gaseous O2 has never been detected
 in the interstellar medium or in other galaxies. Hence it has been suggested
 that the excess O is frozen out in icy grain mantles. Since H2O ice
 contains typically only 10 % of the available elemental O, solid O2 is the
 most likely candidate. Indeed, theoretical models for the composition of
 icy grain mantles predict that O2 can be an important grain mantle molecule
 under some physical conditions. We propose high resolution spectroscopic
 observations with the ISO satellite towards a number of embedded sources
 and southern star-forming regions in order to detect the weak transition
 of solid molecular oxygen and the sharp transition of its photolysis product
 ozone.  Since O2 is a homonuclear diatomic molecule, its vibration and
 rotation do not produce a change in dipole moment, making it IR inactive
 and radio quiet. Interaction with neighbour atoms in the solid state
 however can perturb the symmetry of the vibrations, and modes become
 weakly infrared active. The fundamental transition of O2 at 6.4 mu has
 been recently detected in the laboratory. The behaviour of solid oxygen
 and ozone in astrophysically relevant ice mixtures has been extensively
 studied at Leiden Observatory Laboratory in order to make the search for
 these molecules with ISO successful. For our objects which have a typical
 visual extinction of 20 mag, we would be able to detect the solid O2
 feature if the solid O2 abundance is larger than ~ 10 % of the elemental O.
 The detection of solid molecular oxygen and ozone and the estimation of
 their abundances represent exciting tools in infrared astronomy
 and may reveal if oxygen depleted on grains may account for the lack
 of oxygen measured in the gas phase.