NASCENT OXYGEN:  A single oxygen atom, not yet bonded to anything. Extremely powerful oxidizer when harnessed.

NITROGEN:  When combined with chlorine, nitrogen creates chloramines, which do not belong in our pool. Nitrogen can be found in many swimmer wastes (perspiration, suntan oil, hair tonics, etc.) or be introduced by other means.

NON-CHLORINE SHOCK:  A granular form of potassium permonosulfate, used to oxidize materials such as microorganisms, contaminants or chloramines.  

O-RING: A loop of elastomer with a round (o-shaped) cross-section used as a mechanical seal or gasket. They are designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal at the interface.

OTO:  Another method of testing for free available chlorine levels in your pool, as in an OTO test kit.

OXIDATION:  The "burning up" of organic waste and compounds in the pool water. It also refers to what you may see on your metal pool surfaces if your water is corrosive. Rust is a form of this kind of oxidation.

OZONE:  The molecule containing three atoms of oxygen; known to be a very powerful sanitizer. Ozone producing equipment creates this molecule by UV radiation or corona discharge generators.