- orthocarbonic acid
- osmolality
- osmolarity
- osmondite
- osmosis
- ossification
- ore briquette
- ore bunker
- ore chute
- ore course
- ore elevator
- ore handling
- ore horizon
- ore mass
- ore mineral
- ore plot
- ore separator
- ore sintering
- ore slag
- ore treating
- ore washer
- orendite
- organic analysis
- organic compound
- organic grease
- organic impurity
- organic moderator
- organic polymer
- organic reaction
- organic solvent
- organic sulphide
- organic sulphur
- organogenic
- organogenous sediment
- organomagnesium
- organophilic bentonite
- organosol
- oriented growth
- orifice method
- origanum oil
- original mineral
- original stratification
- orlentite
- orogenetic
- orogenetic period
- orogenic belt
- orogenic cycle
- orogenic phase
- orsellic
- ortho position
- orthoboric acid
- orthoferrites
- orthoformic acid
- orthogenesis
- orthophosphoric
- orthophyric
- orthopinacoid
- orthorhombic
- orthorhombic system
- orthosilicic
- orthosilicic acid
- orthoxylene
- orticant
- ortlerite
- ostracods
- ottajanite
- out-burn
- outburst
- outcrop spring
- outer harbour
- outgush
- outlay
- outlet
- outlet pipe
- outlet pressure
- outlet tube
- outlier
- outlook
- outsert molding
- outside caliper
- outside pressure
- outside price
- outward freight
- oven aging
- over investment
- over-acidify
- over-ground
- overage dip
- overall composition
- overall length
- overall measurement
- overall reaction
- overall size
- overannealing
- overblowing
- overblown steel
- overcast
- overcure
- overcuring
- overdeeping