Ornithomimus has a difficult past. There are many reasons for this. The most prominent reason is the number of specimens that have gotten confused with other animals in the past and the arguments that surround the confusion of naming animals. Regardless, the first species, the type species was named by Marsh in 1890. He named this first species Ornithomimus velox based on a partial foot and hindlimb found in 1889 near Denver, Colorado. Velox is Latin meaning swift. Marsh turned around in the same year and named two more species of Ornithomimus: O. grandis and O. tenius. These species both consisted of fragments that would later be assigned to tyrannosaurid remains. Two years later he named another Hatcher discovery from Wyoming O. sedens and then, not a moment later, named another animal from a different set of fragmentary evidence calling it O. minutus.
Marsh had made things confusing at the gate for these quick animals by naming three species on three fragmentary sets of remains. To be honest, Marsh was a prolific name giver and had been at his trade for a good while. He was confident and, like many later 19th Century scientists, was caught up in the romance of discovering an ancient world. Lawrence Lambe, who had been studying animals in Canada for years as well named a sixth species about 14 years after Marsh's naming whirlwind had settled down calling his remains O. altus. Henry Fairfield Osborn corrected Lambe a decade later by reclassifying the remains as Struthiomimus remains. Between 1920 and 1933 four more scientists would name or rename at least 6 different animals as Ornithomimus species, one of which was O. edmonticus, a one of the only three accepted species names (though one is questioned). It seemed as if the Ornithomimus craziness started by Marsh would never end!
Along came Dale Russell. In 1967 Russell added two more Ornithomimus from the ranks of Struthiomimus and it seemed as though the naming craze was back for the old dinosaurs. However, after a five year period of research and study, Russell came back with proof and research that showed how Struthiomimus and Ornithomimus could be, once and for all, accurately placed and named. His morphometrical study showed that the two genera could be separated based on statistical differences in a few proportions of the skeleton. Struthiomimus and Ornithomimus could be differentiated and accepted as two separate and valid genera. Confusion still remained about remains within the genera and many of the names of the past became invalid and seen as synonyms or tossed out completely. In 1979 the final species was added after renaming Leidy's 1868 New Jersey tibia pair from Coelosaurus antiquus to Ornithomimus antiquus. In 1997 this was reconsidered as an elder synonym to both the velox and edmonticus species and in 2004 was proclaimed defunct by David Weishampel.