- option code
- option field
- option instruction
- Option key
- option list
- option switch
- option table
- optional
- optional facilities
- optional field
- optional file
- optional hyphen
- optional indicator
- optional list
- optional pause
- optional sampling
- optional set
- optional stop
- optional suppression
- optional switch
- optional word
- options menu
- optomechanical mouse
- or
- or circuit
- OR element
- OR function
- OR operation
- OR relationship
- or-gate
- oracle
- orbit
- orbital electron
- orbital insertion
- orbital period
- orbital stability
- orbital topology
- ORC font
- order
- order entry
- order relation
- order sequence
- order set
- order tape
- order type
- order wire
- order-preserving
- ordered
- ordered aggregate
- ordered array
- ordered closure
- ordered domain
- ordered field
- ordered group
- ordered list
- ordered pair
- ordered ring
- ordered search
- ordered set
- ordered tree
- Ordering
- ordering bias
- ordering relation
- ordering set
- orderly shutdown
- ordinal number
- ordinal product
- ordinal type
- ordinale
- ordinary point
- ordinary token
- orgware
- oricycle
- orientable manifold
- orientation variables
- orientation-reversing
- oriented circle
- oriented graph
- oriented sphere
- origin
- origin system
- original language
- original source
- originate
- originate mode
- originator
- originator indication
- orogenesis cycle
- orphan
- orthocenal circles
- orthocomplement
- oscillogram
- osculating circle
- osculating cubic
- osculating helicoid
- osculating plane
- osmotic permeability
- ordinary
- ordinary case
- ordinary derivative