OD Abbreviation for outside diameter.

Operating pressure Pressure at which a system of sprinklers operates, usually measured at the base or nozzle of a sprinkler.

Opportunity time Time that water inundates the soil surface with opportunity to infiltrate.

Orchard valve Outlet valve installed inside a short vertical pipe (riser) with an adjustable cover or lid for flow control.  Similar to an alfalfa valve, but with lower flow capacity.  Typically used in basin irrigation.

Orifice Opening with a closed perimeter through which water flows.  Certain shapes of orifices are calibrated for use in measuring flow rates.

Osmotic potential Potential attributable to the presence of solutes in the soil- in other words, to the soil solution. 

Oven dry Refers to soil samples that have been dried in an oven at 105 C for 24 hours ... 

Overlap Area which is watered by two or more sprinklers.