The N_J = 3₃ – 1₂ line of molecular oxygen at 487249.270 (30) MHz has been detected tentatively with the SWAS satellite:
P. F. Goldsmith, D. Li, E. A. Bergin, G. J. Melnick, V. Tolls, J. E. Howe, R. L. Snell, and D. A. Neufeld,
Tentative Detection of Molecular Oxygen in the ρ Ophiuchi Cloud
Astrophys. J. 576, 814–831 (2002).
A column density of 3.17 (62) × 10¹⁶cm^–2 was derived. Since the column density of H₂ is rather uncertain, the fractional abundances of O₂ with respect to H₂ should be compared with great caution.
Since this is the only putative O₂ line that has been observed, this detection should be viewed with great caution ! Moreover, the line occurs at a local standard of rest velocity of 6.0 km s^–1, almost 3 km s^–1 different from that of many other molecules. This deviation corresponds to more than 4 MHz !
At any rate, the study demonstrates an amount of O₂ in the ISM that is considerably lower than predicted by most models. The amount would drop even more should the detected line turn out to be not due to O₂.

More recently, the N_J = 1₁ – 1₀ line at 118750.340 (7) MHz was reported as a 5σ line toward the same source:
B. Larsson, R. Liseau, L. Pagani, P. Bergman, P. Bernath, N. Biver, J. H. Black, R. S. Booth, V. Buat, J. Crovisier, C. L. Curry, M. Dahlgren, P. J. Encrenaz, E. Falgarone, P. A. Feldman, M. Fich, H. G. Florén, M. Fredrixon, U. Frisk, G. F. Gahm, M. Gerin, M. Hagström, J. Harju, T. Hasegawa, Å. Hjalmarson, L. E. B. Johansson, K. Justtanont, A. Klotz, E. Kyrölä, S. Kwok, A. Lecacheux, T. Liljeström, E. J. Llewellyn, S. Lundin, G. Mégie, G. F. Mitchell, D. Murtagh, L. H. Nordh, L.-Å. Nyman, M. Olberg, A. O. H. Olofsson, G. Olofsson, H. Olofsson, G. Persson, R. Plume, H. Rickman, I. Ristorcelli, G. Rydbeck, A. A. Sandqvist, F. V. Schéele, G. Serra, S. Torchinsky, N. F. Tothill, K. Volk, T. Wiklind, C. D. Wilson, A. Winnberg, and G. Witt,
Molecular Oxygen in the ρ Ophiuchi Cloud
Astron. Astrophys. 466, 999–1003 (2007).
The observations were carried out with the Odin satellite. A column density of 1 × 10¹⁵cm^–2 was derived, considerably lower than in the paper above. This indicates that the 487 GHz line is very likely caused by a different, currently unknown species. The relative abundance to H₂ is, as rather commonly, very uncertain. It has been estimated as 5 × 10^–8. As this is the only O₂ line that has been observed with some certainty, the detection of O₂ should still be viewed with a bit of caution.

A total of three lines, N_J = 3₃ – 1₂, 5₄ – 3₄, and 7₆ – 5₆, at 487249.264 (17), 773839.512 (48), and 1120714.821 (31) MHz, respectively, were recently reported by:
P. F. Goldsmith, R. Liseau, T. A. Bell, J. H. Black, J.-H. Chen, D. Hollenbach, M. J. Kaufman, D. Li, D. C. Lis, G. Melnick, D. Neufeld, L. Pagani, R. Snell, A. O. Benz, E. Bergin, S. Bruderer, P. Caselli, E. Caux, P. Encrenaz, E. Falgarone, M. Gerin, J. R. Goicoechea, Å. Hjalmarson, B. Larsson, J. Le Bourlot, F. Le Petit, M. De Luca, Z. Nagy, E. Roueff, A. Sandqvist, F. van der Tak, E. F. van Dishoeck, C. Vastel, S. Viti, and U. Yildiz,
//Herschel// Measurements of Molecular Oxygen in Orion
Astrophys. J. 737, Art. No. 96 (2011).
Even though the telescope was pointed toward the H₂ Peak 1 position with emission lines much weaker in most cases than in the Hot Core region and some 27“ away from it, the LSR velocity seems to suggest an association with a region called “Peak A” in the course of ammonia observations in 1983, and it is just outside the Hot Core region. The region is likely warm, 80 to 100 K, possibly even warmer. A column density of 6.5 (10) × 10¹⁶cm^–2 was derived. The estimate of the abundance relative to H₂ was of order of 10^–6. Warm dust, rather than shocks, were inferred to explain the fairly high abundance.

R. Liseau, P. F. Goldsmith, B. Larsson, L. Pagani, P. Bergman, J. Le Bourlot, T. A. Bell, A. O. Benz, E. A. Bergin, P. Bjerkeli, J. H. Black, S. Bruderer, P. Caselli, E. Caux, J.-H. Chen, M. de Luca, P. Encrenaz, E. Falgarone, M. Gerin, J. R. Goicoechea, Å. Hjalmarson, D. J. Hollenbach, K. Justtanont, M. J. Kaufman, F. Le Petit, D. Li, D. C. Lis, G. J. Melnick, Z. Nagy, A. O. H. Olofsson, G. Olofsson, E. Roueff, Aa. Sandqvist, R. L. Snell, F. F. S. van der Tak, E. F. van Dishoeck, C. Vastel, S. Viti, and U. A. Yildiz,
reported on,
Multi-line Detection of O₂ toward ρ Ophuichi A
Astron. Astrophys. 541, Art. No. A73 (2012).
The HIFI instrument on board of the Herschel satellite was usd to study lines at 487.2 GHz and at 773.8 GHz. This investigation extends the one performed with the Odin satellite mentioned above and secures that identification.

Contributor(s): H. S. P. Müller; 12, 2003; 02, 2006; 04, 2007; 08, 2011; 05, 2012