- oblique impact
- oblique notching
- oblique-angled
- obliterate
- obliterated corner
- obscuring window
- observation
- observation period
- obsolescence
- obstacle effect
- obstruction
- obturation
- obturator
- occupancy standards
- occupational classification
- occupational taxes
- occupied space
- ocean curing
- ocean greyhound
- ocean tramp
- oceanic retreat
- oceanwards
- ocher
- octagonal pier
- octal
- octant
- odeum
- off plumb
- offlap
- offset printing
- ogee
- ogee crest
- ogee molding
- ogee roof
- ogive
- oil canning
- oil corrosion
- oil gas
- oil separator
- oil varnish
- oil-bath
- oil-drilling
- oiler
- old excavation
- olympic games
- omission
- on edge
- on site
- optimal
- optimal humidity
- optimal reliability
- optimal scale
- optimal system
- optimality criterion
- optimization
- optimum coupling
- optimum density
- optimum method
- optimum slippage
- optimum solution
- optimum speed
- option
- option menu
- opus
- opus incertum
- opus latercium
- opus mixtum
- opus quadratum
- orange peel
- oratory
- orbicular structure
- orchestra platform
- order
- order architecture
- order wire
- ordinance
- ordinary wave
- ordnance surveyor
- ore bin
- ore crusher
- ore dressing
- ore truck
- ore unloading
- ore washing
- organic clay
- organic content
- organic growth
- organic loading
- organisation chart
- oriel window
- orient
- orientate
- orientation
- oriented structure
- orienting screw
- original ground
- original income
- originate
- originating traffic
- ornament