Colombia is the largest producer of oil palm in the Americas. However, the impacts of the expansion of this crop on native fauna remain unclear. Using camera trap records, we evaluated the effect of oil palm and other landscape and local factors on richness and composition of mammals in the Eastern plains or Llanos Orientales of Colombia. We detected 26 species in total (plus 3 with direct observation) of which 17 species were shared between both land use/cover. Oil palm richness was on average 47% lower than forest, and species composition varied greatly between sites and species. Apart from jaguarundis and foxes that were more abundant in oil palm, and giant ant eaters that displayed similar (high) abundances in both forest and oil palm, other species had significantly lower abundances in oil palm plantations compared to native forest. The most important driver for richness and composition at the landscape scale was land use/cover, for which oil palm had a strong negative relation. Cattle and undergrowth vegetation had an important negative and positive influence, respectively, for richness and composition inside oil palm plantations, whereas no vegetation covariates inside forests emerged as important drivers for diversity. Oil palm plantations are expected to expand in this region of Colombia, and our results suggest that improvement of habitat complexity inside the plantations as well as maintenance and restoration of secondary forest is important in maintaining the diversity of mammalian species enhancing the conservation value of this human-dominated landscape.