OBLIGATE. Necessary, an organism that can only live off another living organism is an obligate parasite.

OBTUSE. A blunt or rounded leaf tip.

ORAL TOXICITY. The relative toxicity of a chemical when introduced through the mouth.

ORIFICE. Opening in a nozzle tip, duster, or granular applicator through which a pesticide flows.

OSMOSIS. An interchange of materials between two substances that are separated by a semipermeable membrane.

OVERSEEDING (winter). The seeding of stands of warm season grasses with cool season species to maintain a green turf through the dormant period of the warm season grass.