O-Ring

Round rubber washer used to create a watertight seal, chiefly around valve stems.

OD

Outside Diameter

OEM

Original Equipment Manufacturer.

Oakum

Loosely woven hemp rope that has been treated with oil or other waterproofing agent; it is used to caulk joints in a bell and spigot pipe and fittings.

Oil Powered

A water heater that uses oil as its fuel source.

One-Piece Toilet

A toilet in which the tank and bowl are manufactured as a single vitreous china fixture. One-piece toilets usually have a lower profile than two-piece toilets.

Operating Cost

The cost of running a water heater for a given time period.

Orbital Weld

A circumferential, full fusion weld used to join together two lengths of tubing. It is a GTAW welding process. Usually to join coiled lengths.

Organic Matter

Plant and animal residues, or substances made by living organisms.

Orifice

An opening through which gas is discharged and whereby the flow of gas is limited and/or controlled.

Outlet / Discharge

The opening through which the water exits the pump.

Outlet Sewer

Pipe section in a septic system which runs between the septic tank and the drainage field.

Outside Diameter

The diameter of a pipe measured from the outside edge. (OD)

Overflow hood On a bath drain, the decorative hood concealing the overflow.

Overflow Tube

The vertical tube inside a toilet tank that directs water into the bowl in case the ballcock malfunctions. It is usually part of the flush valve. It prevents potential water damage caused by a tank overflow. A "constant-running" condition alerts the user to a problem. On most toilets, the overflow tube also has a refill tube flowing into it, which directs water from the ballcock through the overflow tube to the bowl, after a siphon break.

Ozone

An unstable form of oxygen (O₃ ), which can be generated by an electrical discharge through air or regular oxygen. It is a strong oxidizing agent and has been used in water conditioning as a disinfectant.