- O
- O'Nan group
- O-minimal
- O-minimality
- O-notation
- O-symbol
- Oban number
- Obelisk
- Obelus
- Object
- Object language
- Objective function
- Oblate ellipsoid
- Oblate spheroid
- Oblate spheroidal coordinates
- Oblateness
- Oblique Angle or Oblique angle
- Obliquity of the ecliptic
- Oblong number
- Observability
- Obstruction
- Obtuse angle
- Obtuse triangle
- Occam's Razor or Occam's razor
- Occultation
- Octachoron
- Octacontagon
- Octadecagon
- Octagon
- Octagonal number
- Octagonal prism
- Octagram
- Octahedral group
- Octahedral number
- Octahedron
- Octahemioctahedron
- Octakaidecagon
- Octal
- Octant
- Octave
- Octiamond
- Octillion
- Octodecillion
- Octomino
- Octonion
- Octothorpe
- Odd function
- Odd number
- Odd number theorem
- Odd order theorem
- Odd Perfect Number or Odd perfect number
- Odd permutation
- Odd prime
- Oddest prime
- Odds
- Odds ratio
- ODE or Ode
- Odious number
- Of the form
- Ogive
- Olga Ladyzhenskaya
- Olga Oleinik
- OLS
- Omega constant
- Omega Function or Omega function
- Omega limit set
- Omega-consistency
- Omino
- Onduloid
- One or ONE
- One and only one
- One way function
- One-factorization
- One-form
- One-parameter subgroup
- One-point compactification
- One-sheet hyperboloid
- One-sheeted hyperboloid
- One-sided derivative
- One-sided derivatives
- One-sided ideal
- One-sided limit
- One-to-one
- One-to-one correspondence
- One-way function
- One-way hash function
- Onto
- Open
- Open and closed maps
- Open ball
- Open book decomposition
- Open cover
- Open disk
- Open interval
- Open map
- Open mapping
- Open mapping theorem
- Open neighborhood
- Open problems
- Open set
- Open-closed set
- Open-mapping theorem
- Operad
- Operand
- Operation
- Operational calculus
- Operations Research or Operations research
- Operator
- Operator norm
- Operator ring
- Operator spectrum
- Operator Theory or Operator theory
- Operator topology
- Opposite number
- Opposite ring
- Opposition
- Optical Illusion or Optical illusion
- Optimal control
- Optimal Golomb Ruler or Optimal Golomb ruler
- Optimization
- Optimization theory
- Optimum
- Option pricing
- Opus number
- Or (logic)
- Oracle
- Orbifold
- Orbison's illusion
- Orbit
- Orbit-counting theorem
- Order
- Order (graph theory)
- Order and degree of polynomial
- Order ideal
- Order isomorphic
- Order of magnitude
- Order of operations
- Order relation
- Order statistic
- Order statistics
- Order topology
- Order type
- Ordered field
- Ordered group
- Ordered list
- Ordered pair
- Ordered ring
- Ordered set
- Ordered vector space
- Ordering
- Ordinal
- Ordinal addition
- Ordinal exponentiation
- Ordinal multiplication
- Ordinal number or Ordinal Number
- Ordinal space
- Ordinary Differential Equation or Ordinary differential equation
- Ordinary generating function
- Ordinate
- Ore condition
- Ore domain
- Ore number
- Ore's theorem
- Orientable manifold
- Orientable surface
- Orientation
- Orientation-preserving
- Orientation-reversing
- Oriented Graph or Oriented graph
- Oriented surface
- Origami
- Origin
- Orlicz norm
- Orlicz space
- Orlicz-Sobolev space
- Ornstein isomorphism theorem
- Ornstein's theorem
- Ornstein-Uhlenbeck process
- Orr-Sommerfeld equation
- Orthic circle
- Orthic triangle
- Orthocenter
- Orthocentre
- Orthocentric system
- Orthocomplemented lattice
- Orthodrome
- Orthogonal
- Orthogonal Array or Orthogonal array
- Orthogonal basis
- Orthogonal complement
- Orthogonal functions
- Orthogonal group
- Orthogonal Latin squares or Orthogonal latin squares
- Orthogonal matrix
- Orthogonal polynomials
- Orthogonal projection
- Orthogonal projector
- Orthogonal set
- Orthogonal subspaces
- Orthogonal system
- Orthogonal trajectory
- Orthogonal transformation
- Orthogonal vectors
- Orthogonality
- Orthogonality relation
- Orthogonality relations
- Orthogonality theorem
- Orthogonalization
- Orthographic projection
- Orthomodular lattice
- Orthonormal basis
- Orthonormal set
- Orthonormal transformation
- Orthonormal Vectors or Orthonormal vectors
- Orthonormality
- Orthoplex
- Orthotomic
- Osborne's rule
- Oscillating differential equation
- Oscillation
- Osculate
- Osculating circle
- Osculating curves
- Osculating plane
- Osculation
- Ostrowski theorem
- Ostrowski's theorem
- Otto Hölder
- Outcome
- Outdegree
- Outer automorphism
- Outer automorphism group
- Outer measure
- Outer product
- Outer regular
- Outlier
- Outplanar graph
- Outside
- Oval
- Ovals of Cassini
- Over
- Overbar
- Overdamping
- Overdetermined system
- Overdispersion
- Overdot
- Overhand knot
- Overline
- Overload
- Oversampling
- Ovoid
