optical rotatory poweroptical stabilityoptically activeoptimizationoptimum temperatureoptimum valueorbitorbitalorbital overlaporderorder of magnitudeoreore depositeore flotationorganic chemistryorganic magnetorganic pesticidesorganic polymerorganometallic compoundorientationorientation forcesorificeorifice meterorsat apparatusortho directorortho para directorortho positionorthogonal functionsorthogonal setorthogonalityorthogonality theoremorthohydrogenorthonormal functionsorthorhomobic systemososazoneoscillatoroscillogramoscillographoscillometryoscilloscopeosmiumosmometerosmosisosmoticosmotic pressureostwald viscosimeterout of phaseouter shellouter shell electron