A room temperature-curable organopolysiloxane composition, which is capable of giving a cured silicone rubber having excellent resistance against oils, e.g., automobile engine oil, and little responsible for foaming of the oil contaminated with the silicone rubber, can be prepared by first blending a silanol-terminated dimethyl polysiloxane, an iminoxy-substituted organosilane compound, e.g., vinyl tributanoxime silane, an organotin compound as a catalyst and an inorganic filler. e.g, zinc oxide, to give a uniform mixture and further admixing the uniform mixture with a trimethylsilyl-terminated dimethyl polysiloxane, an alkenyloxy-substituted organosilane compound, e.g., vinyl triisopropenyloxy silane, and a methyl polysiloxane having a network structure of the molecules as consisting of the monofunctional units of the formula (CH.sub.3).sub.3 SiO.sub.0.5 and tetrafunctional units of the formula SiO.sub.2 each in a specified amount.