Ocellularia diminuta is a corticolous (bark-dwelling), crustose lichen in the family Graphidaceae. The species was described in 2014 from primary rainforest near Porto Velho in Rondônia, Brazil, where it grows on tree bark in shaded understory conditions. It forms olive-green patches with a slightly bumpy surface and produces small, rounded fruiting bodies that often lack a central column. The lichen is distinguished from similar species by its smaller size and shorter ascospores, and remains known only from its original collection locality.
The species was described in 2014 by Marcela Cáceres, André Aptroot, and Robert Lücking as part of a survey of Graphidaceae in Rondônia, Brazil. The holotype was collected in March 2012 at the Parque Natural Municipal de Porto Velho on tree bark in primary rainforest; it is housed at ISE (Herbário da Universidade Federal de Sergipe).
In their the authors compared Ocellularia diminuta with O. papillata: the new species has smaller, rounded, -lacking (ecolumellate) ascomata, whereas O. papillata has larger apothecia with a developed columella. They further noted that the only other ecolumellate species in the genus with an uncarbonized and small, transversely septate ascospores is O. albocincta, which differs in having a much thicker, whitish thallus, almost immersed ascomata, and longer ascospores (about twice as long).
The thallus is corticolous and epiperidermal, up to about 5 cm across, with a surface that is smooth but bears scattered to dense, low bumps and short ridges; it is olive-green and lacks a visible . In section the thallus is 30âÂÂ50 üm thick, with a cortex 5âÂÂ10 üm thick, a 15âÂÂ20 üm thick, and a 20âÂÂ30 üm medulla containing numerous small crystals and clusters of calcium oxalate. The Trentepohlia has cells roughly 7âÂÂ11 à6âÂÂ9 üm.
The apothecia are rounded to slightly irregular and few in number, often absent from much of the thallus; they are with a complete , measuring 0.3âÂÂ0.5 mm in diameter and 0.15âÂÂ0.2 mm high. The is covered by a 0.05âÂÂ0.07 mm pore, so the disc is not visible externally; the is entire, prosoplectenchymatous, yellowish, and 15âÂÂ20 üm wide; a columella is absent. The is 90âÂÂ100 üm high and clear, with a 5âÂÂ10 üm epithecium that is indistinct; the is hyaline. Paraphyses are unbranched with smooth tips; are absent.
Ascospores (numbering eight per ascus) are ellipsoid, 5-septate, 12âÂÂ15 à5âÂÂ6 üm (2âÂÂ3 times as long as wide), hyaline, and with lens-shaped ; in iodine they stain I+ violet-blue (amyloid). No secondary metabolites were detected by thin-layer chromatography; the medulla is PâÂÂ, and microscopic section is KâÂÂ.
The species is known from primary rainforest in Rondônia, Brazil, where it grows on smooth tree bark in shaded understory conditions near Porto Velho at around 100 m elevation. As of 2025, it has not been recorded elsewhere in Brazil.