An optical matching oil that is immune to timewise variations in its refractive index, that undergoes little increase in viscosity below room temperature, and that also resists crystallization below room temperature. The optical matching oil is based on an organosilicon compound with the following general formula ##STR1## wherein R is selected from the group consisting of methyl, vinyl, and 2-methylphenethyl with the proviso that at least one R group is the 2-methylphenethyl group.