oo ringoblique latticeoblique systemobolete processobservabilityobservable quantityocclusionocheroctagonoctahedraloctahedral holesoctahedronoctane numberoctet ruleodd electronoil bathoil paintolefinolefin copolymerolefin metathesis reactionolefin resinoleumoligomeroligomerizationoligosaccarideolive oilomnidirectionalone component systemone electron atomone particle eigenfunctionone way chromatographyonsetopaqueopen chain compoundopen systemoperatoropiumoptical activityoptical antipodeoptical densityoptical isomeroptical isomerismoptical lensoptical purityoptical pyrometryoptical resolutionoptical resonatoroptical rotationoptical rotatory dispersion