Formulas
- Oppermann's conjecture
- Orbit (dynamics)
- Order of operations
- Order theory
- Order type
- Ordinal definable set
- Orientation (vector space)
- Orientation character
- Ornstein–Uhlenbeck operator
- Oscillation theory
- Out(Fn)
- Outer space (mathematics)
- Ramanujan–Nagell equation
- Modulus and characteristic of convexity
- Morera's theorem