Scuba & Nitrox Glossary: O
Nitrox and breathing-gas terminology is prioritized first. General scuba terms follow below. Definitions are educational summaries and do not replace formal training, manufacturer instructions, or an operational dive plan.
Nitrox & Gas Terms — O
- O₂
- Chemical symbol for molecular oxygen.
- Oxygen Analyzer
- An instrument used to determine oxygen concentration in a breathing gas.
- Oxygen-Compatible Air
- Air meeting specified purity requirements for use in preparing oxygen-enriched mixtures; the applicable standard must be identified.
- Oxygen Clean
- A condition achieved through procedures that remove contaminants that could promote ignition in elevated-oxygen service.
- Oxygen Clock
- A display or calculation used to track oxygen exposure over time.
- Oxygen Exposure
- The combined effect of oxygen partial pressure and duration.
- Oxygen Fraction
- The proportion of oxygen in a gas mixture.
- Oxygen Service
- Equipment preparation, materials, lubrication, handling, and cleanliness suitable for specified oxygen exposure.
- Oxygen Toxicity
- Harmful effects from excessive oxygen exposure, primarily discussed as central-nervous-system and pulmonary toxicity in diving.
- OTU
- Oxygen tolerance unit, used to estimate cumulative pulmonary oxygen exposure.
- O2 Clean Label
- A marking indicating equipment has been prepared for specified oxygen service.
- O2-Compatible Lubricant
- Lubricant selected for oxygen service under relevant manufacturer guidance.
- Offgassing Nitrogen
- Release of absorbed nitrogen from tissues as pressure decreases.
- Oxygen Backfill
- Adding oxygen or another gas in a planned sequence to complete a blend.
- Oxygen Sensor Millivolts
- The electrical output used to evaluate many galvanic oxygen sensors.
General Scuba Terms — O
- Octopus
- A common term for an alternate second-stage regulator.
- Open Circuit
- Scuba in which exhaled gas is released into the water.
- Open Water
- An unrestricted natural body of water used for diving or certification.
- Overhead Environment
- An environment where direct vertical access to the surface is blocked physically or by decompression obligation.
- Overweighting
- Carrying more ballast than needed, which can increase drag and complicate buoyancy control.
- Ocean Surge
- Back-and-forth water movement produced by waves.
- Octopus Holder
- A retainer keeping an alternate second stage accessible and protected.
- Offshore Dive
- A dive conducted away from the immediate shoreline, usually from a vessel.
- Open-Water Ascent
- An ascent without a physical line or fixed reference.
- Out-of-Air Signal
- The standard hand signal indicating loss of breathing gas.