Detailed observations have been conducted on the interfacial deformation of a silicone oil surface and a water drop falling on it. Eleven kinds of silicone oils with wide variations of kinematic viscosity, nu(T)=1-10(5) mm(2)/s, have been tested. The oil surface is disturbed by a water drop with a diameter d(L)=3.1 mm, which falls freely on it from a height of 100-1000 mm. Special attention has been directed to the maximum depth of the cavity formed on the oil surface D(M) and to the maximum diameter of the water drop spreading on the oil surface d(M). We have categorized the configurations of the oil cavity into seven patterns, and those of the water drop at the oil-water interface into five patterns. The maximum cavity depth D(M)/d(L) can be well correlated by a dimensionless group Re(TL)We(TL), where Re(TL) is Reynolds number based on d(L) and nu(T) and We(TL) is Weber number with the water density and surface tension of oil. The maximum diameter of the impacting water drop d(M)/d(L) can be correlated by the Reynolds number with a viscosity of water (Re(L)) and the Ohnesorge number (Oh). Moreover, the condition under which the impacting water drop is smashed into pieces has been also examined based on Re(L) and Oh.