This South American rodent has a dark coat. A survey in central Brazil captured most of its recorded animals in wet grassland.
A species identification key in the rodent volume describes the coat of O. delator as blackish.
At Chapada dos Veadeiros, a survey captured 25 animals: 22 in vereda wetland grassland and three in campo úmido, or wet grassland.
An extinction-risk assessment has not been added yet.