- objective
- objective reality
- objectivity
- obligatory well
- oblique bridge
- oblique dovetail
- oblique fault
- obliquity
- obscure
- obscured glass
- observation data
- observation grid
- observation ground
- observation panel
- obsidian
- obtuse arch
- occupancy
- occupational representation
- occupy
- ocean liner
- octagonal dome
- octagonal spire
- octahedral
- office building
- office treatment
- officework
- official financing
- offset bend
- offset footing
- offset joints
- offset line
- offset paper
- offset section
- offshoot
- ogee arch
- oil burner
- oil catcher
- oil derrick
- oil geology
- oil groove
- octagonal tile
- octangular
- octastylos
- oculus
- oermanent guy
- off-center
- off-line
- offensive
- office
- office block
- office chief
- office computation
- office furniture
- office regulations
- offset
- offstream storage
- ogee curve
- ogee spillway
- oil asphalt
- oil leak
- oil mud
- oil stain
- oil tar
- oil trap
- oiliness
- oily impregnation
- old working
- oleo-resins
- oneness modulus
- onion dome
- oil scraper
- oiled canvas
- oilfield
- oily
- ok button
- oleo-resin
- oleoresinous paint
- omnkey engine
- on strike
- one thing
- onramp
- onyx marble
- oolite
- opacity
- opal glass
- opal rock
- opaque
- opaque enamel
- opaque glass
- opaque glaze
- open
- open area
- open bidding
- open boarding
- open branching
- open bridge
- open caisson
- open canal
- open channel
- open cornice