It is a medium-sized species, weighing about 45 g (1.6 oz), with mostly grayish and brownish fur and short and broad hindfeet with well-developed pads.

An American Museum of Natural History expedition led by Sydney Anderson collected the first three specimens of Oecomys sydandersoni in 1964 and 1965.

[5] In 2009, Michael Carleton, Louise Emmons, and Guy Musser described the latter as a new species, Oecomys sydandersoni, referring to it the specimens collected in the 1960s and previously identified as O. concolor.

[6] They named the new species after Sydney Anderson in honor of his work on the Bolivian mammal fauna, including the first collection of O. sydandersoni.

Its short, soft, and fine fur is bright ochraceous brown to pale tawny on the upperparts, changing moderately abruptly into the generally gray underparts.

[14] The skull shows a short front part (rostrum) and a broad interorbital region (located between the eyes).

[14] O. sydandersoni is known from the departments of Beni and Santa Cruz in eastern Bolivia, including the Noel Kempff Mercado National Park (NKMNP), where the type locality is located.

[17] All but a few specimens come from pockets of woodland in seasonally flooded grasslands, where it is the most frequently encountered rodent; the related oryzomyine Hylaeamys acritus, the spiny rat Proechimys longicaudatus, and the opossum Marmosa murina were found in the same habitat.

It joins several other species with restricted ranges found in the NKMNP, including Hylaeamys acritus, the akodontine rodents Juscelinomys guaporensis and J. huanchae, and an opossum, Cryptonanus unduaviensis.