The OH radical was the first molecule whose detection by means of radio astronomy was reported in 1963.
S. Weinreb, A. H. Barrett, M. L. Meeks, and J. C. Henry
announced
Radio Observations of OH in the Interstellar Medium
Nature 200, 829–831 (1963).
They used the 25.6 m dish of the Millstone Hill Observatory of Lincoln Laboratory (MIT) to identify the two strongest hyperfine structure (HFS) components of the ²Π_3/2 fundamental (J = 1.5 − 1.5) Λ-doubling transition at 1667.359 and 1665.402 MHz in absorption toward the supernova remnant Cassiopeia A, aka 3C 461 or GAL 111.7-02.1.

J. G. Bolton, K. J. van Damme, F. F. Gardner, and B. J. Robinson
carried out
Observation of OH Absorption Lines in the Radio Spectrum of the Galactic Centre
Nature 201, 279 (1964).
They employed the Parkes 64 m dish to observed the same lines toward Sagittarius A.

F. F. Gardner, B. J. Robinson, J. G. Bolton, and K. J. van Damme
reported on the
Detection of the Interstellar OH Lines at 1612 and 1720 Mc/sec
Phys. Rev. Lett. 15, 3−5 (1964).
These weaker HFS components were measured with the same dish as above on the basis of improved spectroscopic parameters prior to their measurements in the laboratory.

H. Weaver, D. R. W. Williams, N. H. Dieter, and W. T. Lum
carried out
Observations of a Strong Unidentified Microwave Line and of Emission from the OH Molecule
Nature 208, 29−31 (1965).
They used the 25.9 m dish of the Hat Creek Observatory for OH observations toward several sources. OH was found in emission toward the high mass star-forming regions W49 and NGC 6334.

B. Zuckerman, P. Palmer, H. Penfield, and A. E. Lilley
announced the
Detection of Microwave Radiation from the ²Π_1/2, //J// = 1/2 State of OH
Astrophys. J. 153, L69−L76 (1968).
The NRAO 43 m dish was employed to detect the F = 1 − 0 HFS component toward the high mass star-forming regions W3 and W49.

B. Zuckerman, J. L. Yen, C. A. Gottlieb, and P. Palmer
reported on
Observations of the ²Π_3/2, //J// = 5/2 State of Interstellar OH
Astrophys. J. 177, 59−78 (1972).
Using the same dish, the strongest F = 3 − 3 HFS component toward the high mass star-forming regions W3(OH), W75N, W49, Sgr B2(OH), and NGC 6334 N, as well as toward the O-rich late-type star NML Cyg. The weaker F = 2 − 2 component was detected toward W3(OH) and NGC 6334 N.

J. W. V. Storey, D. M. Watson, and C. H. Townes
published
Detection of Interstellar OH in the Far-Infrared
Astrophys. J. 244, L27−L30 (1981).
The Kuiper Airborne Observatory was employed to detect the fundamental rotational transition of OH (²Π_3/2 J = 2.5 − 1.5) near 2.51 THz toward Sgr B2.

Some time later,
R. Genzel, D. M. Watson, M. K. Crawford, and C. H. Townes
investigated
The Neutral-Gas Disk around the Galactic Center
Astrophys. J. 297, 766−786 (1985).
The Kuiper Airborne Observatory was employed to observe also the low energy rotational transition of OH (²Π_1/2 J = 1.5 − 0.5) near 1.836 THz.

K. E. S. Ford, D. A. Neufelf, P. F. Goldsmith, and G. J. Melnick
announced the
Detection of OH toward the Extreme Carbon Star IRC +10216
Astrophys. J. 589, 430−438 (2003).
The 305 m Arecibo dish was used to detect the two strongest HFS components of fundamental Λ-doubling transition near 1665 and 1667 MHz.

J. R. Goicoechea, J. Cernicharo, M. R. Lerate, F. Daniel, M. J. Barlow, B. M. Swinyard, T. L. Lim, S. Viti, and J. Yates
published on
Far-Infrared Excited Hydroxyl Lines from Orion KL Outflows
Astrophys. J. 641, L49−L52 (2006).
Several rotational transitions of OH in both spin-ladders as well as between them were detected with ISO-LWS.

T. Khouri, L. Velilla-Prieto, E. De Beck, W. H. T. Vlemmings, H. Olofsson, B. Lankhaar, J. H. Black, and A. Baudry
described the
Detection of Highly Excited OH towards AGB Stars — A New Probe of Shocked Gas in the Extended Atmospheres
Astron. Astrophys. 623, Art. No. L1 (2019).
Employing ALMA, Λ-doubling transitions in v = 0 up to N = 18 were detected toward W Hya and R Dor; transitions in v = 1 up to N = 17 were detected toward W Hya.

F. F. Gardner, R. X. McGee, and M. W. Sinclair
detected
¹⁸OH in Sagittarius
Astrophys. Lett. 5, 67−71 (1970).
The two strongest HFS components of the fundamental Λ-doubling transition were observed with the Parkes 64 m dish toward Sgr A and Sgr B2. The ¹⁶O/¹⁸O rotio was ~314 and ~203, respectively, somewhat lower than ~500 terrestrial.

F.F. Gardner and J.B. Whiteoak
published on
The Detection of Interstellar ¹⁷OH
Mon. Not. R. Astron. Soc. 176, 57P−60P (1976).
Similar observations led to the detection in Sgr A. The ¹⁸O/¹⁷O rotio was 4.2 ± 0.4, again slightly smaller than ~5.5 terrestrial.

B. Parise, F. Du, F.-C. Liu, A. Belloche, H. Wiesemeyer, R. Güsten, K. M. Menten, H.-W. Hübers, and B. Klein
announced the
Detection of OD towards the Low-Mass Protostar IRAS 16293−2422
Astron. Astrophys. 542, Art. No. L5 (2012).
The GREAT receiver on board SOFIA was employed for the observations. A relatively high ratio of OD to HDO was found.

Contributor(s): H. S. P. Müller; 2026; 07.