Tridiminished Icosahedron
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Johnson Solid 
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- Orthogonal Vectors
- Orthographic Projection
- Orthologic
- Orthonormal Basis
- Orthonormal Functions
- Orthonormal Vectors
- Orthopole
- Orthoptic Curve
- Orthotomic
- Orthotope
- Osborne's Rule
- Oscillation
- Oscillation Land
- Osculating Circle
- Osculating Curves
- Osculating Interpolation
- Osculating Plane
- Osculating Sphere
- Osedelec Theorem
- Ostrowski's Inequality
- Ostrowski's Theorem
- Otter's Tree Enumeration Constants
- Outdegree
- Outer Automorphism Group
- Outer Product