The analysis of the interfacial wave properties is an important point in understanding of many aspects of separated-flow patterns (annular and stratified). One may cite flow pattern stability, pressure drop and heat transfer as characteristics affected by the wave properties. Previous studies have shown that the phenomenon of flow pattern transition in stratified flow can be related to the interfacial wave structure (problem of hydrodynamic instability). The study of the wavy stratified flow pattern requires the characterization of the interface, i.e., average wave shape, wave speed, amplitude and wavelength as a function of flow properties. Studies on waves in stratified liquid–liquid flow are scanty, even more when related to viscous oils. This article offers new experimental data on interfacial waves collected in a glass test line of 12 m and 0.026 m i.d., oil (density and viscosity of 854 kg/m3 and 0.3 Pa s at 20 C, respectively) and tap water as the working fluids; the stratified flow was filmed with a high speed video camera at several inclinations from horizontal ( 5 , 0 , 5 , 10 ). New experimental data and available literature data of interfacial waves in oil–water flow were collected, analyzed and correlated to the flow properties by dimensionless numbers of Reynolds, Froude and Weber. A second-order Fourier series is proposed to model the wave shape. The correlations can be used to predict the average wave geometry and wave speed of typical oil–water interfacial waves within a significant range of superficial velocities and pipe inclinations. Considering the simplicity of the proposed correlation, the agreement between data and predicted wave is encouraging.