O-RING:

A loop of elastomer in a round "O" shape that is used as a mechanical seal or gasket. They fit in a groove and are compressed during the assembly of two or more parts, which creates a seal at the interface.

 

OTO: 

A way to test for free available chlorine levels in a swimming pool. Usually comes in the form of a 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.