O
- Occupancy Sensors
- Ocean Energy Systems
- Ocean Thermal Energy Conversion (OTEC)
- Ocean Thermal Energy Conversion (OTEC)
- Octane
- Octane Rating
- Off Peak
- Off-highway Use
- Off-Hours Equipment Reduction
- Off-Peak Gas
- Off-site Produced Energy for Heat, Power, and Electricity Generation
- Off-system
- Offshore Oil and Gas Operations
- Offshore Reserves and Production
- Offsystem (Natural Gas)
- Ohm (Ω)
- Ohm's Law
- Oil
- Oil Company Use
- Oil Formation Volume Factor (FVF)
- Oil from Algae
- Oil Reservoir
- Oil Shale
- Oil Stocks
- Oil Well
- Oil Well (Casing Head) Gas
- Old Field
- Old Reservoir
- Olefinic Hydrocarbons (Olefins)
- On-highway Use (Diesel)
- On-Peak
- On-site Generation for Own Use
- On-System Energy
- On-System Sales
- One Sun
- One-time Fee
- Onsite Transportation
- Onsystem (Natural Gas)
- Open Access
- Open Access (Electric)
- Open Access Transmission Tariff (OATT)
- Open Market Coal
- Open Refrigeration Unit
- Operable Capacity
- Operable Generators/Units
- Operable Nuclear Unit (Foreign)
- Operable Nuclear Unit (U.S.)
- Operable Refineries
- Operable Unit
- Operable Utilization Rate
- Operated (Oil and Gas)
- Operating Capacity
- Operating Day
- Operating Expenses
- Operating Income
- Operating Revenues
- Operating Subsidiary
- Operating Unit
- Operating Utilization Rate
- Operational Readiness and Process Governance (OPRG)
- Operations and Maintenance (O&M)
- Operator, Gas Plant
- Operator, Oil and/or Gas Well
- Optional Delivery Commitment
- Order
- Organic Content
- Organic Waste
- Organisation for Economic Co-operation and Development (OECD)
- Organization for Economic Cooperation and Development (OECD)
- Organization of Petroleum Exporting Countries (OPEC)
- Organization of the Petroleum Exporting Countries (OPEC)
- Original Cost
- Original Equipment Manufacturer (OEM)
- Original Equipment Manufacturer (OEM)
- Original Equipment Manufacturer Vehicle (OEM Vehicle)
- Original Gas-in-Place (OGIP)
- Original Oil-in-Place (OOIP)
- Other (Miscellaneous Category)
- Other Agricultural and Forestry Products
- Other Capital Costs
- Other Demand-Side Management (DSM) Assistance Programs
- Other End Users
- Other Energy Losses
- Other Energy Operations
- Other Finished Products in Petroleum
- Other Fuel Alcohol
- Other Gas
- Other Generation
- Other Hydrocarbons
- Other Industrial Plant
- Other Load Management
- Other Oils Equal to or Greater Than 401 Degrees Fahrenheit
- Other Operating Costs
- Other Oxygenates
- Other Power Producers
- Other Recycled Feed and Waste
- Other Refiners
- Other Renewable Fuels
- Other Service to Public Authorities
- Other Single-Unit Truck
- Other Supply Contracts
- Other Trucks/Vans
- Other Unavailable Capability
- Other Vegetable Oils
- Other Waste Oils, Fats, or Greases
- Outage
- Outer Continental Shelf (OCS)
- Outer Continental Shelf (OCS)
- Output
- Oven
- Overburden
- Overburden Ratio
- Overriding Royalty (ORR)
- Owned Reserves
- Owned/Rented
- Owner Occupied
- Owner's Equity
- Ownership
- Ownership of Building
- Oxidize
- Oxygenated Gasoline
- Oxygenates
- Ozone (O₃)
- Ozone Precursors (NOx and VOCs)