The first firm detection of of oxidaniumylidene or hydroxylium, OH⁺, was reported by
P. P. van der Werf, K. G. Isaak, R. Meijerink, et al.,
Black Hole Accretion and Star-Formation as Drivers of Gas Excitation and Chemistry in Markarian 231
Astron. Astrophys. 518, (2010) Art. No. L42.
The account is somewhat sketchy, no rest frequencies are given and no references for these. An abundance has been derived, but no details are given. Only a broad overview spectrum is shown. Thus, one may be inclined to view this detection with some caution. On the other hand, all three fine structure components of the N = 1 – 0 transition seem to have been observed in emission toward the ultraluminous infrared galaxy Mrk 231with correct relative intensities employing the SPIRE instrument on board of the Herschel satellite. Given the ubiquity and high abundance of OH⁺ in Galactic sources, the identification is probably correct.

There have been prior reports on the tentative detection of OH⁺:
E. González-Alfonso, H, A. Smith, J. Fischer, and J. Cernicharo,
The Far-Infrared Spectrum of Arp 220
Astrophys. J. 613, (2004) 247–261
discuss the possibility that absorption features of NH observed with ISO LWS at 100 and 150 μm may be affected by the N = 3 – 2 and 2 – 1 transitions, respectively, of OH⁺. However, they refrain from suggesting that there is strong evidence for OH⁺ to be present.
E. González-Alfonso, H. A. Smith, M. L. N. Ashby, J. Fischer, L. Spinoglio, and T. W. Grundy,
High-excitation OH and H₂O Lines in Markarian 231: The Molecular Signatures of Compact Far-infrared Continuum Sources
Astrophys. J. 675, (2008) 303–315
assigned two barely detected (about 2.5 σ) absorption features detected near 150 μm with ISO LWS to the two strongest fine structure components of the N = 2 – 1 transition.
Detailed modeling may provide clues as to how plausible these observations are.

E. González-Alfonso, J. Fischer, S. Bruderer, et al.
reported on
Excited OH⁺, H₂O⁺, and H₃O⁺ in NGC 4418 and Arp 220,
Astron. Astrophys. 550, Art. No. A25 (2013);
using PACS on board of the Herschel satellite. Transitions up to N = 4 – 3 with upper state energy of almost 500 K were detected in both sources. There may be even evidence for the 5 – 4 components (E_up ≈ 700 K) in NGC 4418, but the intensities do not fit the model.

Contributor(s): H. S. P. Müller; 10, 2010; 04, 2013.