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E. olivalis is a member of the detersa group of the subgenus Euxoa. They are medium-sized moths (3.0-3.4 cm wingspan) with brown or grey-brown forewings, longitudinally streaked with black, light grey and olive brown. The costa and cubital vein are contrasting light grey, especially near the base. The elongated orbicular spot and crescent shaped reniform are also outlined and partially filled with contrasting pale grey scales. There is a thin black basal line, and black scales outline the claviform spot and fill the cell before the orbicular and between the orbicular and reniform spots. The veins are mostly lined with black scales, and there is a series of basally pointing black arrowheads between the veins in the submarginal area. The terminal band is dark brown. There is usually a slightly paler olive or yellow-brown streak running from below the orbicular spot to the anal angle. Hindwings pale buff usually shading to smoky brown on the outer third; entirely brown in females and some males. Very similar to E. plagigera, and to a lesser extent to E. oblongistigma. The light grey or white cubital vein will separate most specimens of olivalis from these similar species. Questionable specimens may require dissecting for positive identification. Lafontaine has prepared useful keys to the detersa group (Lafontaine, 1981) and to the genus Euxoa (Lafontaine, 1987).

From southern Saskatchewan south to west to south-central BC, south to southern California, northern Arizona and New Mexico. In Alberta, it has been collected in the mountains, parklands and grasslands, from just north of Jasper and the Red Deer area south.