We present a literature review devoted to the study of a craniovertebral malformation such as the os odontoideum of C2 vertebra. This is a rare anomaly that often results in atlantoaxial instability and, as a consequence, a wide range of clinical manifestations. The review is analytic and was conducted using medical literature databases and search resources, namely, PubMed (MEDLINE), Google Scholar, and eLibrary. The review focused on the definition of os odontoideum and its epidemiology, including the structure of craniovertebral junction pathologies. Features of the embryonic development of the craniovertebral junction, which are important in terms of the notion of the theories of the origin of os odontoideum, are described. The anatomical variants of the os odontoideum are presented, and a wide range of clinical manifestations of this pathological condition and approaches to its diagnosis are described.