O. arnoldii was first described from Tarragona, Spain by Delforge in 1999 and is named in honour of the contemporary botanist, Mr J. P. Arnold. This is a member of the O. funerea group of Pseudophrys, though there is little consensus amongst experts as to the true status of the species and it is currently the subject of an ongoing study involving it and other Western Mediterranean Pseudophrys such as O. lupercalis and O. forestieri.

As with all the Pseudophrys of Northern Spain and Southern France the distribution of this species is not fully understood, it is however known to be common and sometimes abundant in Northern and Eastern  Spain together with Southern France as far north as Bouches-du-Rhone. It is also present in the Balearic Islands.

O. arnoldii will tolerate a range of conditions from full sun to mid shade and dry to moist soils, though always on calcareous substrates. It is a robust plant with flowers that are large and often numerous (up to ten) with lateral lobes that although separated from the median lobe by significant sinuses, are invariably folded under in a way that usually disguises this feature. This species readily hybridizes and intermediate swarms are common. O. arnoldii is a late flowerer and this can be a useful, if not diagnostic differentiating feature. It seldom appears before mid April and can be found in exceptional conditions as late as July.

The pictures come from various locations in Eastern and Northern Spain and date from the last week in April, at which time the plants were just beginning their flowerng cycle.