Apart from "O. edmontonicus" dating to the early Maastrichtian, two other species are presently considered to be possibly valid, both from the late Maastrichtian. "O. sedens" was named by Marsh in 1892 from partial remains found in the Lance Formation of Wyoming, only one year after the description of "O. velox". Dale Russell, in his 1972 revision of ornithomimids, could not determine which genus it actually belonged to, though he speculated that it may be intermediate between "Struthiomimus" and "Dromiceiomimus". In 1985 he considered it a species of "Ornithomimus". Although it has since been referred to mainly as "Struthiomimus sedens", based on complete specimens from Montana (as well as some fragments from Alberta and Saskatchewan), these yet have to be described and compared to the "O. sedens" holotype.