The interfingering Baruungoyot and Nemegt Formations of Mongolia host an exceptional diversity of oviraptorosaurs that is unique in including members of all three Late Cretaceous families (avimimids, caenagnathids, and oviraptorids). The oviraptorosaurs from the Baruungoyot and Nemegt Formations are reviewed, with a description of a new species of Avimimus. Emended diagnoses are provided for each taxon, including the first diagnosis of Rinchenia mongoliensis. The anatomy of these oviraptorosaurs is updated, taking into account data from recent discoveries. Avimimids and caenagnathids may be monotaxic in the Nemegt Formation, but oviraptorids are represented by at least four genera, with one additional undescribed genus. Oviraptorosaur ecology is examined by incorporating them into a dataset of nearly 500 dinosaur occurrences throughout the Nemegt Region. Oviraptorosaurs compose a small but constant part of Nemegt faunas. Oviraptorid species each occur at a single locality, which may be an artifact of sampling. Alternatively, it may indicate that they were spatially partitioned, or that species turned over rapidly. The occurrence of avimimids and caenagnathids throughout the Nemegt Formation and their absence in the Baruungoyot Formation suggests that they preferred mesic habitats. In contrast, oviraptorids were successful in both xeric and mesic environments, but are relatively more abundant in the former. The coexistence of these families can be explained through dietary niche partitioning. Oviraptorosaurs probably had an Asian origin, but rapidly spread to North America and interchanged between continents several times in the Late Cretaceous.