，合同变换群- orthocenter
- orthocentric
- orthocentric centroid
- orthocentric coordinate
- orthocentric line
- orthocentric quadrangle
- orthocentric quadrilateral
- orthocentric system
- orthocentric tetrahedron
- orthocentric triangle
- orthocentroidal circle
- orthocentroidal sphere
- orthochronous Lorentz group
- orthocompactness
- orthocomplement
- orthocomplementation
- orthocomplemented lattice
- orthocomplement space
- orthoconvexity
- orthocorrespondent
- orthocubic
- orthodiagonal quadrangle
- orthodox
- orthodox semigroup
- orthodrome