Fl. Bor.-Amer. 2: 174. 1803.
Plants loosely cespitose; rhizomes long. Culms obtusely trigonous in cross-section, 30–90 cm, smooth or scabrous-angled distally. Leaves: basal sheaths reddish purple; ligules as wide as long to wider than long; blades dark green, wiry, filiform, with involute margins, 0.5–2.5 mm wide, glabrous. Inflorescences 3–20 cm; proximal bract 3–15 (–25) cm, shorter than to exceeding inflorescence; proximal 1–2 spikes pistillate, erect, ovoid to cylindric; terminal 1 (–2) spikes staminate. Pistillate scales ovate, 3.2–5.6 × 2.6–3.8 mm, shorter than perigynia, margins entire, apex obtuse to acute, awnless. Perigynia ascending, 7–15-veined, veins extending into beak, ovate, 4–6.7 × 2.5–3.4 mm, leathery, apex tapered; beak 0.3–0.9 mm, obscurely bidentulate, glabrous, teeth straight to somewhat outcurved, to 0.3 mm. Stigmas 3. Achenes brown, trigonous, smooth.
Phenology: Fruiting summer.
Habitat: Bogs, often forming extensive stands in Sphagnum-dominated areas, poor fens, sometimes in acidic, sandy, or peaty soils in open swamps, marshes, lakeshores, riverbanks
Elevation: 0–1200 m
Man., N.B., Nfld. and Labr., N.S., Ont., Que., Sask., Yukon, Ind., Maine, Mich., Minn., N.H., N.Y., N.C., Ohio, Pa., Vt., Wis.
Carex oligosperma hybridizes rarely with C. rostrata.