- orthopositronium
- orthoprism
- orthoprojector
- orthoptic
- orthoptic circle
- orthoptic curve
- orthoptics
- orthoptic sphere
- orthoptic triangle
- orthoptist
- orthopyramid
- orthopyroxene
- orthopyroxenite
- orthoradioscopy
- orthorectifier
- orthorhombic
- orthorhombic crystal system
- orthorhombic system
- orthoscope
- orthoscopic
- orthoscopic eyepiece
- orthoscopic image
- orthoscopicity
- orthoscopy
- orthoselection