The
thallus of
O. fenestrata is
crustose and grows on bark. It is olive-green, continuous, and up to across, with a thickness of about 60–100
μm. It has a dense with a smooth to uneven surface texture. The is from the
green algal genus
Trentepohlia, with cells about 7–12 × 5–8 μm. Crystals are generally absent from the except near the
apothecia, and the
medulla is indistinct and white. The fruiting bodies (
apothecia) are rounded to angular, sunken in the thallus to partly protruding (immersed to ), and about 0.4–0.6 mm in diameter. The is covered by a pore-like opening (about 0.2–0.3 mm wide) and a central column (columella) that ranges from simple to irregularly perforated (fenestrate). The columella is not blackened (not ) and is colorless to brown toward the top. The outer wall of the fruiting body () is light brown to brown, and hair-like filaments () are absent. The spore-bearing layer (
hymenium) is clear and about 90–100 μm high. Each
ascus contains eight oval (
ellipsoid), 6-celled (5-septate)
ascospores measuring about 15–20 × 6–8 μm, with thick cross-walls and lens-shaped internal spaces (); they stain violet-blue with iodine (I+ violet-blue). Reported
lichen substances include
psoromic,
subpsoromic, and 2'-
O-demethylpsoromic acids. ==Habitat and distribution==