A hydrosilylation reaction-curable organopolysiloxane composition comprising (A) a methylpheny-lalkenylpolysiloxane that has at least two alkenyl groups wherein diphenylsiloxane units are no more than 5 mole % and at least 20 mole % is comprised of phenyl groups, (B) a methylphenylhydrogenpolysiloxane that has at least two Si-bonded hydrogen atoms wherein diphenylsiloxane units are no more than 5 mole % and at least 20 mole % is comprised of phenyl groups, and (C) a hydrosilylation reaction catalyst. An optical semiconductor element sealant comprising this composition. An optical semiconductor device sealed with this optical semiconductor element sealant.