Plants perennial, densely cespitose, knotty based, 20-40 cm; rhizomes absent. Culms leafy at base, filiform, wiry. Leaves: blades filiform, nearly reaching inflorescence tip or much shorter, to 0.3 mm thick, apex tapering. Inflorescences: spikelet clusters mostly 2-6, simple or reduced to 1 spikelet, often with 2 capillary branches, one divaricate or reflexed, 1 ascending; 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 narrowly ovate, 3-5(-6) mm, apex acute, midrib included or short excurrent. Flowers: perianth bristles 6, not reaching past fruit midbody, stubby, plumose to near tip. Fruits 3-8 per spikelet, 2-2.5 mm; body light brown to brown, ellipsoid-obovoid, tumidly lenticular, 1.5-2 × 1.6-1.7 mm; surfaces faintly, interruptedly cross rugulose, apically indented under tubercle; tubercle low conic, 0.5 mm, base flaring, circular.

Fruiting spring-summer. Moist to wet sands or peats of bogs, depressions in savannas, open pinelands, pond shores; 0-200 m; Ala., Fla., Ga., La., Miss., N.C., S.C. Va.; West Indies.

Rhynchospora breviseta is sympatric with R. oligantha over much of its range; intergrades have not been seen.