- one-way infinite Turing machine
- one-way layout
- one-way trapdoor function
- online analysis
- online (= on-line)
- online system
- on-off element
- Ono inequality
- Ono Takash
- O-notation
- Onsager differential equation
- Onsager, L.
- Onsager reciprocity relation
- onto
- ontological
- ontology
- open
- open-and-closed set
- open-and-closed subset
- open arc
- open ball
- open base
- open cell
- open circle
- open-closed set