Plants perennial, densely cespitose, knotty based, 20–40 cm; rhizomes absent. Culms filiform, leafy at base, wiry. Leaves ascending to erect; blades filiform, nearly terete, or channeled, sometimes compressed, nearly reaching distal inflorescence or much shorter, 0.2–0.3 mm thick, apex subulate. Inflorescences: spikelet clusters 2–6, simple or reduced to 1 spikelet, branches ascending to divaricate or reflexed; leafy bracts single per cluster, filiform, setaceous, with clusters appearing lateral to bracts. Spikelets pale red brown, ellipsoid lanceoloid, 5–6(–8) mm, apex acute to acuminate; fertile scales oblong elliptic, convex, acuminate, 3.5–5 mm, apex broadly acute, midrib forming apiculus. Flowers: perianth bristles 6, reaching to or slightly past tubercle base, increasingly plumose from middle to base. Fruits 1–3 per spikelet, (2.5–)2.7–3(–3.4) mm; body light brown to brown, ellipsoid obovoid, distally conspicuously necked, tumidly lenticular, 1.7–2.5 × 1.5–1.8 mm; surfaces smooth or minutely transversely rugulose; tubercle conic subulate, 0.5–0.7 mm, base flaring.

Fruiting spring–summer. Sands and peats of bogs, depressions in savannas, open pinelands, seeps; 0–200 m; Ala., Del., D.C., Fla., Ga., La., Miss., N.J., N.C., S.C., Tex., Va.; West Indies; Central America.

Rhynchospora oligantha is distinguished from other taxa of its complex mostly by the distinctive neck at the achene apex, a feature essentially absent in R. breviseta, its closest relative. Those two species have been heavily impacted by conversion of pine savannas to cropland or pine plantations; even with abandonment or clearing of such land, they are very slow to reoccupy the disturbed sites.