Description: Thallus crustose, rather thick (0.5-2 mm), smooth to wrinkled or areolate, dull, whitish, ochre-grey or pale brown, forming patches to 5 cm across (usually less). Apothecia lecanorine, round to (more rarely) irregular, 1-3 mm across, with a flat, pink-ochre, finely white-pruinose disc and a very thick, smooth, prominent thalline margin. Cortex of the thalline margin well developed, in upper part up to 70 μm, with a well differentiated medulla; proper exciple well visible in section, superficially often forming a ring around disc; hymenium colourless, 250-300 μm tall; paraphyses thin, densely branched and anastomosed; hypothecium colourless to yellowish. Asci (4-)6-8-spored, with thick, amyloid walls, Pertusaria-type. Ascospores 1-celled, hyaline, broadly ellipsoid, (35-)45-70(-75) x (12-)25-40 µm. Photobiont: chlorococcoid. Thallus: K-, C- or C+ yellow, KC+ yellow, P-; disc of apothecia KC+ carmine red. Chemistry: gyrophoric and lecanoric acid in the epithecium, variolaric acid in thallus, occasionally with the murolic acid complex and alectoronic acid.
Note: a mainly temperate species found on deciduous trees in humid areas. The relationships with O. parella remain to be clarified: the two species are similar, but have a different ecology and distribution, and they hardly can be treated as formae of one and the same species.
Growth form: Crustose
Photobiont: green algae other than Trentepohlia
Reproductive strategy: mainly sexual
Most common in areas with a humid-warm climate (e.g. most of Tyrrenian Italy)