The study investigates the flow characteristics and propulsive efficiency of oscillating foils through force and power measurements, as well as visualization data. The experiments were conducted at Reynolds numbers of 1100 and 40,000, using a NACA 0012 foil with a chord length of 10 cm and a span of 60 cm. The foil was subjected to harmonic heave and pitch motions, with various combinations of heave amplitude-to-chord ratio, pitch amplitude, phase angle, and Strouhal number. The results show that high propulsive efficiency, up to 87%, can be achieved under optimal wake formation conditions. Visualization data reveal that the formation of a moderately strong leading-edge vortex on alternating sides of the foil, convected downstream and interacting with trailing-edge vorticity, results in the formation of a reverse Kármán street, which is crucial for high efficiency. The phase angle between transverse oscillation and angular motion is identified as the critical parameter affecting the interaction of leading-edge and trailing-edge vorticity and the efficiency of propulsion. The experimental findings are compared with theoretical predictions from linear and nonlinear inviscid theory, showing good agreement over a certain parametric range.The study investigates the flow characteristics and propulsive efficiency of oscillating foils through force and power measurements, as well as visualization data. The experiments were conducted at Reynolds numbers of 1100 and 40,000, using a NACA 0012 foil with a chord length of 10 cm and a span of 60 cm. The foil was subjected to harmonic heave and pitch motions, with various combinations of heave amplitude-to-chord ratio, pitch amplitude, phase angle, and Strouhal number. The results show that high propulsive efficiency, up to 87%, can be achieved under optimal wake formation conditions. Visualization data reveal that the formation of a moderately strong leading-edge vortex on alternating sides of the foil, convected downstream and interacting with trailing-edge vorticity, results in the formation of a reverse Kármán street, which is crucial for high efficiency. The phase angle between transverse oscillation and angular motion is identified as the critical parameter affecting the interaction of leading-edge and trailing-edge vorticity and the efficiency of propulsion. The experimental findings are compared with theoretical predictions from linear and nonlinear inviscid theory, showing good agreement over a certain parametric range.