*o*(*f*),*O*(*f*) - see Functions - Theory - Limit - Order of functions- oblique asymptote - see Derivatives - Theory - Graphing functions - Asymptotes
- odd function - see Functions - Theory - Real functions - Basic properties and Functions - Methods Survey - Basic properties - Symmetry
- odd functions and Fourier series - see Series - Theory - Series of functions - Fourier series
- odd functions and Taylor series - see Series - Theory - Series of functions - Taylor series
- one-to-one mapping - see Extra - Sets and mappings - Mappings
- 1-1 function
- one-sided continuity - see continuity
- one-sided derivative - see Derivatives - Theory - Introduction - Derivative and Derivatives - Methods Survey - Derivative
- one-sided limit - see Functions - Theory - Limit - Definition
- onto, function - see Functions - Theory - Real functions - Introduction
- onto mapping - see Extra - Sets and mappings - Mappings
- open set - see Functions - Theory - Real numbers - Topological notions
- operations with Fourier series - see Series - Theory - Series of functions - Properties of Fourier series
- operations with functions - see Functions - Theory - Real functions - Operations
- operations with power series - see Series - Theory - Series of functions - Power series
- operations with sequences - see Sequences - Theory - Introduction - Operations, and limit: Sequences - Theory - Limit - Properties
- operations with sequences of functions - see Series - Theory - Series of functions - Sequences of functions
- operations with series of functions - see Series - Theory - Series of functions - Series of functions
- operations with series of real numbers - see Series - Theory - Introduction - Basic properties
- operations with Taylor series - see Series - Theory - Series of functions - Taylor series
- optimization - see Derivatives - Theory - Applications - Global extrema, optimization and Derivatives - Methods Survey - Applications - Global extrema, optimization
- order of evaluation of a function - see this note
- order of expressions (sequences) - see Sequences - Theory - Limit - Intuitive evaluation and this note
- order of functions (comparison of growth) - see Functions - Theory - Limit - Order of functions and Functions - Theory - Limit - Intuitive evaluation for application to limits
- outer function - see composition in Functions - Theory - Real functions - Operations
- outside function - see composition in Functions - Theory - Real functions - Operations