The etiology of os odontoideum has been debated in the literature since the condition was initially described. The authors present the case of a 4-year-old girl who was found to have an os odontoideum with atlantoaxial instability after a motor vehicle collision. Imaging performed 3 years earlier demonstrated an incompletely ossified, cartilaginous, orthotopic os separated from the body of the odontoid process at the level of the secondary ossification center with a short odontoid process. This case presents the earliest imaging demonstration of the presence of a congenital orthotopic os odontoideum at the secondary ossification center. The authors review the pertinent literature and propose that the etiology of os odontoideum is multifactorial and related to the embryology and vascular supply to the odontoid process.