Fruiting body annual, effused-reflexed, sessile, elongate fan-shaped, 3.0-7.0 cm long, 2.0-3.0 cm wide, 0.5-1.0 cm thick; margin wavy, sometimes over-lapping the tube layer; upper surface white, tomentose, becoming matted in age, azonate, bruising or weathering tawny-brown to vinaceous-brown, eventually dark rusty-brown; context when young soft, white, resilient, up to 0.5 cm thick, discoloring like the upper surface, in age rigid, brownish; odor slightly pungent; taste somewhat bitter.
Pores 3-4 per mm, whitish, round to angular, bruising vinaceous-brown to reddish-brown, dissepiments thin, becoming torn to dentate with age; tubes 4.0-5.0 mm long, whitish, bruising like the pores.
Spores 4.0-5.0 x 1.0-1.5 µm, cylindrical in face view, sausage-shaped in side-view, smooth, thin-walled, with prominent granular inclusions, hilar appendage not obvious, inamyloid, not dextrinoid; spores white in deposit.
Solitary or in small groups, imbricate or in horizontal lines on conifer logs and branches; fruiting from mid to late winter.
This polypore was once placed in the genus Tyromyces. We have followed Gilbertson and Ryvarden who transferred it to Oligoporus on the basis of the rot it forms, brown rather than white. Oligoporus fragilis is a fairly unremarkable polypore: small, whitish, soft-textured when young, with a matted-tomentose upper surface, and annual fruiting habit. Its most obvious feature, bruising reddish-brown, is mimicked by three other polypores, Parmastomyces transmutans, Amylocystis lapponica and Leptoporus mollis. These species can be separated in the field by their tendency to bruised reddish-brown directly rather than through an intermediate tawny-brown stage. With a microscope Parmastomyces transmutans differs in having dextrinoid spores, Amylocystis lapponica by amyloid cystidia, and Leptoporus mollis by the lack of clamped septate, generative hyphae.