Oligacanthorhynchus microcephalus (Rudolphi, 1819) Schmidt, 1972 was originally described from the philander opossum Caluromys philander from Brazil. Oligacanthorhynchus tortuosa (Leidy, 1850) Schmidt, 1972 was originally described from the Virginia opossum (Didelphis virginiana) in North America. Six years after describing O. tortuosa, Leidy indicated the synonomy of O. tortuosa with O. microcephalus. This taxonomic view persisted for decades until Van Cleave, in 1924, recognized the validity of O. tortuosa, apparently without examining the type material of O. microcephalus. Subsequent workers have recognized 2 species of Oligacanthorhynchus from New World opossums: O. microcephalus from South American opossums and O. tortuosa from North American opossums. In this study, comparison of individuals of O. tortuosa from D. virginiana from the United States with the type specimen of O. microcephalus revealed the worms to be conspecific. It was thus determined that O. tortuosa is a junior synonym of O. microcephalus and O. microcephalus is redescribed on the basis of both laboratory-reared material and the type specimen. Additionally, comparison of the type material of Oligacanthorhyncus tumida (Van Cleave, 1947) Schmidt, 1972 revealed these specimens to be conspecific with O. microcephalus; thus O. tumida (Van Cleave, 1947) Schmidt, 1972 is considered a junior synonym of O. microcephalus. Analysis of the literature and museum specimens reveals that O. microcephalus exhibits a broad distribution throughout much of the New World, utilizing an array of hosts with substantially overlapping ranges.