- An angle of less than 90°.锐角：小于九十度的角
- orthodontics
- orthodonture
- orthodox
- Orthodox Church
- orthodoxes
- orthodoxies
- Orthodox Judaism
- orthodox sleep
- orthodoxy
- orthoepies
- orthoepist
- orthoepists
- orthoepy
- orthogenesis
- orthogonal
- orthogonal projection
- orthogonals
- orthographic
- orthographical
- orthographic projection
- orthographies
- orthography
- orthomolecular
- orthopaedics
- orthopedics