2012 OP was originally discovered as a minor planet by R.H. McNaught using the Uppsala 0.5-m Schmidt from Siding Spring Survey on 2012 Jul. 16.62, when it was at mag. 20V. It was also a T3 candidate because of is Tj parameter (-0.98).
It was observed by other southern observatories for 2 months, then it was “lost” in the solar conjunction (2012 Dec. 4).
It was recovered by Hidetaka Sato on 2013, Mar. 20 using remotely a 0.51-m f/6.8 reflector from iTelescope Observatory, Siding Spring (MPC Q62), then it was observed again by himself on Apr. 9 and 11 remotely from iTelescope Observatory, Mayhill (MPC H06) with the same instrumentation.
Hidetaka sent an alert to the T3 mailing-list noting its apparent cometary feature (FWHM larger than stars nearby). After his alert, I successfully observed it from Schiaparelli Observatory on Apr. 15 with a 0.60-m f/4.64 reflector, and I could confirm what Hidetaka found. In a stack of images totalling 30 minutes of exposure time in good conditions, it had a FWHM profile about 20% larger than stars nearby (4.1″ vs 3.4″), but I knew I had to request the attention of a larger telescope…
In the meantime, Minor Planet Center issued 2012 OP recovery with MPEC 2013-H12 on Apr. 17.
On behalf of a larger collaboration I have with IASC team (composed of P. Miller, P. Roche, A. Tripp, R. Miles, R. Holmes, S. Foglia, T. Vorobjov, T. Lister, E. Gomez and myself) I managed to observe it on Apr. 19.48-49 with the 2.0-m f/10 Faulkes Telescope North: a stack of 14×60 seconds images reveals a softer aspect respect to stars, with a coma at least 4″ wide possibly elongated toward the west.
I then alerted both Tim Lister from LCOGT and Bill Ryan from Magdalena Ridge Observatory.
A stack of images taken by Tim with one of their 1.0-m f/8 telescopes from Cerro Tololo (MPC W85) revelaed a clear larger profile respect to stars (2.1″ vs 1.5″), but the definitive evidence came from the terrific images taken by Bill with the MRO 2.4-m f/8.9 reflector. Even in single images a coma and a tail was visible, and a stack of 10×60 seconds images shows a strong central condensation with a 10″ coma and a broad tail 20″ long centered in PA about 250°.
CBAT then issued CBET 3486 (subscription required), though not including all the people involved, on Apr. 23 with the discovery of the cometary nature of this unusual minor planet, again a proof of how fruitful a pro-am collaboration can be. All the astrometry from the discovery until now can be seen on MPEC 2013-H36.
Below the various images/profiles:

(FWHM profiles from H. Sato from Q62, 2013 Apr. 9, larger profile but low SNR)

(my own profiles on Apr. 15, evidence of a larger FWHM)

(T. Lister images from W85, 1.0-m f/8 reflector, here the evidence is much clear)

(FTN image, 14x60seconds, softer aspect and evidence of a coma toward the south-west)

(the definitve confirmation images from Magdalena Ridge Observatory 2.4-m telescope, each frame is a 5×60 seconds, 0.48″/px resolution. Many thanks to the kindness of Bill Ryan).