Wallis Sieve
A compact set 
with Area
created by punching a square hole of length
in the center of a square. In each of the eight squares remaining,punch out another hole of length 
, and so on.- Object
- Oblateness
- Oblate Spheroid
- Oblate Spheroidal Coordinates
- Oblate Spheroidal Wave Function
- Oblate Spheroid Geodesic
- Oblique Angle
- Oblong Number
- Obstruction
- Obtuse Angle
- Obtuse Triangle
- Ochoa Curve
- Octacontagon
- Octadecagon
- Octagon
- Octagonal Number
- Octagram
- Octahedral Graph
- Octahedral Group
- Octahedral Number
- Octahedron
- Octahedron 5-Compound
- Octahemioctacron
- Octahemioctahedron
- Octakaidecagon