The base of the fruit in our species is prolonged into a pointed appendage covered with antrorse bristles that enable the fruit to stick tenaciously to socks and other clothing, as well as animals, which may serve as agents of dispersal.

1. Inflorescence lacking involucels (bractlets at base of the umbellets); flowers all bisexual.

2. Mature fruits clearly concave-tapered to a short beak-like apex; young fruits with at least a slight constriction beneath the truncate apex.

O. berteroi

2. Mature fruit ± convex toward apex, which is blunt, not beak-like; young fruit without any subapical constriction, but straight to slightly convex on the sides beneath the truncate apex.

O. depauperata

1. Inflorescence with involucels; flowers both bisexual and staminate.

3. Styles (even in fruit) not over 1.5 mm long; flowers 4-8 (-10) per umbellet; foliage with at most a weak licorice odor.

O. claytonii

3. Styles becoming 2-4 mm long; flowers (including withering staminate ones) (6-) 9-18 per umbellet; foliage and especially root with strong licorice odor when bruised.

O. longistylis