- open cornices
- open cut
- open digging
- open distribution
- open ditch
- open dovetail
- open drain
- open eaves
- open excavation
- open floor
- open frame
- open gird
- open hood
- open jetty
- open line
- open mortise
- open pediment
- open pit
- open porosity
- open profile
- open relief
- open restraint
- open roadstead
- open roof
- open sewer
- open sheeting
- open space
- open split
- open stair
- open string
- open tank
- open tendering
- open traverse
- open trench
- open up
- open valley
- open vent
- open web
- open well
- open wiring
- opening
- opening bridge
- opening crack
- opening document
- opening leaf
- opening light
- opening width
- opening window
- opera
- operable walls
- operated
- operating bridge
- operating budget
- operating characteristic
- operating expense
- operating maintenance
- operating mechanism
- operating part
- operating period
- operating platform
- operating position
- operating potential
- operating pressure
- operating rating
- operating repair
- operating room
- operating safety
- operating speed
- operating system
- operation
- operation control
- operation disjunction
- operation drawing
- operation manual
- operation schedule
- operational calculus
- operational circuit
- operational diagram
- operational equipment
- operational functions
- operational life
- operational range
- operational test
- operative control
- operator
- oppose
- opposite
- opposite direction
- optical axis
- optical center
- optical radiator
- optical receiver
- optical sag
- optical square
- object box
- oblique bedding
- oblique compression
- oblique cutting
- oblique drawing
- oblique grain