Bot. Gaz. 48: 147. 1909.
Herbs, perennial, 0.9–3 dm; from a woody caudex; with a stout taproot. Stems few to several, erect or ascending, sometimes decumbent at base, unbranched or branched, hairs usually retrorse, medium length, ± soft, eglandular, mixed with very short-glandular ones, sometimes with spreading, long, soft ones. Leaves green to purple, linear to lanceolate, 2–7 cm, not fleshy, margins plane, involute, 3–5(–7)-lobed, apex acuminate to acute; lobes spreading, linear to sometimes narrowly lanceolate, apex acute. Inflorescences 2.5–18 × 1–3.5 cm; bracts pale green to yellow-green or pale, dull reddish brown throughout, or proximally so colored but changing gradually to cream or yellowish on distal margins, narrowly to broadly lanceolate, (3–)5–7(–9)-lobed; lobes ascending, linear, long, proximal lobes arising below mid length, central lobe apex obtuse, others acute. Calyces colored as bracts, 10–25 mm; abaxial and adaxial clefts 6–7 mm, 30–60% of calyx length, deeper than laterals, lateral 5–10 mm, 40–50% of calyx length; lobes linear, apex acute. Corollas straight, 21–36 mm; tube 16–20 mm; teeth of abaxial lip often exserted, beak exserted; beak adaxially green, 4.2–5.5 mm; abaxial lip green to purple, distally white, conspicuous, slightly but noticeably pouched, often visible through front cleft, 3–5 mm, 67–100% as long as beak, puberulent; teeth erect, white, 1.8–2.1 mm.
Phenology: Flowering May–Aug.
Habitat: Dry slopes and plains, sagebrush meadows, grasslands, openings in conifer forests.
Elevation: 900–2200 m.
Castilleja oresbia is endemic to eastern Oregon and adjacent Idaho. It is easily confused with both varieties of C. pallescens, which also occur in sagebrush habitats. Castilleja oresbia has longer calyx lobes and softer pubescence than C. pallescens var. pallescens, although some transitional specimens are found. Castilleja oresbia has a combination of longer calyx lobes, longer pubescence, and obscurely nerved bracts, which usually serve to separate it from C. pallescens var. inverta. All three have different, though somewhat overlapping, ranges. Castilleja oresbia occasionally hybridizes with C. peckiana in Grant County, Oregon.