Perennials, 5-30 cm (taproots deep, woody, usually with 1-2 horizontal, spreading, sprouting branch roots, and root crowns bearing woody, branched caudices). Stems erect, simple or branched, glabrous or sparsely tomentose. Leaves mostly cauline (crowded); alternate; sessile; blades 1- or weakly 3-nerved, linear to narrowly oblanceolate or lanceolate (little reduced to immediately proximal to heads), margins entire, faces glabrous or sparsely villous. Heads radiate or discoid, borne singly or (2-12) in loose corymbiform or nearly glomerate arrays. Involucres hemispheric to campanulate or cylindro-turbinate, 7-25 × 5-20 mm. Phyllaries 20-35 in 3-6 series, 1-nerved (sometimes weakly keeled by thin, indurate midnerves), ovate-triangular to oblong-lanceolate, unequal to subequal, outer herbaceous, inner chartaceous, margins not scarious, flat to convex (apices long-acuminate to abruptly obtuse and cuspidate or apiculate), faces glabrous or puberulent. Receptacles shallowly convex, barely pitted, epaleate. Ray florets 0 or 6-25, pistillate, fertile; corollas yellow. Disc florets 15-50, bisexual, fertile; corollas yellow, tubes shorter than funnelform throats, lobes 5, erect, deltate to triangular; style-branch appendages triangular. Cypselae (brownish) prismatic or narrowly turbinate, slightly compressed to subcylindric, 5-7 mm, 5(-8)-ribbed, faces strigose to scabrous or glabrous; pappi persistent, of 15-30, brownish, flattened, unequal, ciliate-barbellate, apically attenuate bristles in 1-2 series. x = 5 (4). Oönopsis is characterized by a relatively low habit, woody taproots with ample vegetative reproduction via one or two horizontal, spreading, sprouting branch roots, root crowns bearing woody, branched caudices, even-sized cauline leaves continuing up the stems and subtending the heads, foliaceous outer phyllaries, and yellow rays (in two species). All taxa are selenium-accumulating and indicators of selenium substrates. An undescribed fifth species has been noted (G. K. Brown 1993) from Pueblo and Fremont counties, Colorado; it is closely related to Oönopsis foliosa.