- ortho-Right angle, perpendicular.
- orthoepyThe study of pronunciation.
- OrthogonianA member of the Orthogonian society, a social club at Whittier College founded by Richard M. Nixon in opposition to the wealthy elites at Whittier.
- orthomorphicconformal
- orthopediaThe branch of medicine which deals with the prevention and correction of deformities in bodies; orthopedics.
- orthotonosmuscular rigidity associated with tetanus