It is endemic to southern California, where it is known only from the Peninsular Ranges of San Diego County. Its range may extend north into Riverside County. It is a plant of the chaparral slopes and woodlands of the mountains.Astragalus oocarpus is a rare species of milkvetch known by the common names San Diego milkvetch and Descanso milkvetch.
Astragalus oocarpus is a perennial herb producing upright to erect hollow stems up to 1.3 meters tall. Leaves are up to 17 centimeters long and are made up of veiny lance-shaped leaflets each up to 3 centimeters in length. The stem and leaves are mostly hairless.
The inflorescence holds up to 75 cream-colored flowers, each between 1 and 2 centimeters in length. The fruit is an inflated legume pod 1.5 to 2.5 centimeters long which dries to a stiff papery texture.