A curable organopolysiloxane composition and an optical semiconductor element sealant, each comprising (A) a diorganopolysiloxane that has at least 2 alkenyl groups wherein at least 70 mole % of all the siloxane units are methylphenylsiloxane units and the total content of 1,3,5-trimethyl-1,3,5-triphenylcyclotrisiloxane and 1,3,5,7-tetramethyl-1,3,5,7-tetraphenylcyclotetrasiloxane is no more than 5 weight %, (B) an organopolysiloxane that has at least 2 silicon-bonded hydrogen atoms wherein at least 15 mole % of the silicon-bonded organic groups are phenyl groups, and (C) a hydrosilylation reaction catalyst. An optical semiconductor device in which an optical semiconductor element within a housing is sealed with the cured product from the aforementioned composition.