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.- Outer Automorphism Group
- Outer Product
- Oval
- Oval of Descartes
- Ovals of Cassini
- Overlapping Resonance Method
- Oversampling
- Ovoid
- Paasche's Index
- Packing
- Packing Density
- p-adic Number
- Padovan Sequence
- Padé Approximant
- Padé Conjecture
- Painlevé Property
- Painlevé Transcendents
- Pair
- Paired t-Test
- Pair Sum
- Paley Class
- Paley Construction
- Paley's Theorem
- Palindrome Number
- Palindromic Number