Diagnosis: Males similar to other members of Nothobranchius taeniopygus species-group, differing from all other species of Nothobranchius by caudal and anal fins having broad pale sub-marginal band and dark margin (Ref. 86819). Nothobranchius oestergaardi differs from members of N. taeniopygus species-group by the following combination of characters: proximal portion of caudal fin plain red, vs. proximally irregularly spotted in N. boklundi, N. hassoni, N. kafuensis, N. polli, N. rosenstocki, N. symoensi and N. taenipygus or caudal fin covered with spots in N. brieni and N. malaissei; dorsal fin without coloured margin, vs. black fin margin in N. rubroreticulatus or broad light blue fin margin in N. brieni, N. polli and N. symoensi; body without distinct black spots anteriorly, vs. black spots present in N. rosenstocki and N. taeniopygus; anal fin striped, vs. anal fin irregularly striped in N. hassoni and N. polli, spots concentrated on proximal portion in N. brieni, N. kafuensis, and N. malaissei, or anal fin completely marbled in N. rosenstocki and N. symoensi; distal portion of anal fin cream then light blue, vs. yellow in N. boklundi, N. polli, pale yellow to yellow white in N. taeniopygus, light blue or orange in N. kafuensis, light blue in N. rubroreticulatus and red in N. rosenstocki; and with broad dark brown border, vs. narrow dark border in N. boklundi, N. brieni, N. kafuensis, N. polli, or light blue margin in N. symoensi; distal portion of pelvic fin plain light blue, vs. spotted in N. boklundi N. brieni, N. kafuensis and N. polli, plain red in N. rosenstocki, black in N. rubroreticulatus and N. taeniopygus (Ref. 86819). Nothobranchius oestergaardi males with relatively shorter caudal peduncle compared to N. boklundi, 17.0-21.3 vs. 19.1-26.6% of standard length; shallower and longer caudal peduncle compared to N. rosenstocki, 13.6-16.0 vs. 10.1-12.9% of standard length and 17.0-21.3 vs. 12.5-14.5% of standard length respectively; shallower caudal peduncle compared to N. symoens, 13.6-16.0 vs. 12.8% of standard length; longer predorsal length compared to N. hassoni, 57.5-62.5 vs. 52.8-56.4% of standard length, N. rosenstocki, 57.5-62.5 vs. 48.3-52.9% of standard length, N. rubroreticulatus, 57.5-62.5 vs. 53.3-57.6% of standard length, and N. symoensi, 57.5-62.5 vs. 55.1% of standard length; longer prepelvic length compared to N. brieni, 49.6-52.3 vs. 43.3-45.2% of standard length, N. taeniopygus, 49.6-52.3 vs. 42.1-48.9% of standard length, and N. symoensi, 49.6-52.3 vs. 48.2% of standard length; and longer preanal length compared to N. brieni, 56.2-64.4 vs. 52.9-54.5% of standard length (Ref. 86819).

Description: Robust Nothobranchius with deep and compressed body, maximum observed length in males 31.8 mm standard length; dorsal profile slightly concave to nearly straight on head, convex from nape to end of dorsal fin base; ventral profile convex, slightly concave to nearly straight on caudal peduncle posterior to dorsal and anal fin (Ref. 86819). Snout slightly pointed, mouth directed upwards, lower jaw longer than upper, posterior end of rictus at sema level as or slightly above centre of eye; branchiostegal membrane projecting posteriorly from opercle (Ref. 86819). Dorsal and anal fins located posterior to midbody, rounded, tips with short filamentous rays, dorsal fin tip reaching caudal fin; both fins with papillate contact organs on fin rays; dorsal fin rays 16-17; anal fin rays 16-17; pectoral fin approximately triangular, tip reaching pelvic fin; pelvic fin short, separated at base and tip reaching urogenital papillae; caudal fin subtruncate (Ref. 86819). Scales cycloid, body and head entirely scaled, except for ventral surface of head; scales in median lateral series 25-29 + 3-4 on caudal fin base; transverse row of scales 11; circumpeduncular scales 12 (Ref. 86819). Cephalic squamation pattern variable, some specimens presenting irregular G-type; anterior neuromast series 'open' type; central supra-orbital series separate in shallow grooves, each with two neuromasts; posterior cephalic neuromast series curved with 2, rarely 3, neuromasts; one neuromast on each scale of median longitudinal series (Ref. 86819). Basihyal bone sub-triangular; six branchiostegal rays; vomerine teeth present in small patch; lateral process of post-temporal short; second pharyngobranchial with 1 tooth; interarcual cartilagine short; single anterodorsal process of urohyal; total vertenbrae 27; premaxilla and dentary with many irregularly distributed unicuspid, slightly curved teeth of different sizes and small number of larger teeth in outer row of upper and lower jaws (Ref. 86819).

Colouration: colour in life for males: body and head scales light blue with broad dark red margin, creating reticulated pattern on body and head; snout, frontal and dorsal portion of head red; branchiostegal membrane whitish; dorsal fin grey green to grey yellow with irregular rows of dark red small spots and dots; spots somewhat horizontally coalesced proximally, medially elongated on fin rays, distally denser and smaller; anal fin with cream proximal portion with few red irregular spots, followed by red-brown band, a wider cream band becoming light blue distally and wide dark red-brown margin; pelvic fins light blue, red-brown near base; pectoral fins hyaline with light blue margins; caudal fin opaque red-brown, with light blue sub-distal narrow band and dark red-brown, wider margin; iris golden, with faint black vertical bar through centre of eye (Ref. 86819). Colouration in life for females: body and head scales pale brown, with golden iridescence on scale centre; opercular region silver to golden; abdomen silver to golden; all fins hyaline; iris golden, with faint black vertical bar through centre of eye (Ref. 86819). Preserved colouration in males: body scales light brown, all scales with distinct wide dark red irregular margin; dorsal fin light brown with dark red to brown spots; anal fin pale white proximally with dark red to brown spots, followed by brown stripe, pale cream distal area and pale dark red to brown margin; pelvic fins light brown with dark red spots near base; pectoral fins hyaline; caudal fin dark red, with distinct white sub-marginal band and wide dark brown margin; iris grey-blue (Ref. 86819). Preserved colouration in females: body light brown; opercular and ventral area more cream-yellow; unpaired and paired fins light brown; iris grey-blue (Ref. 86819).