The effect of relative motions at the leading-edge of an oscillating airfoil energy harvester is investigated using an inviscid discrete vortex model (DVM). The DVM incorporates a leading-edge separation criterion based on the modified leading-edge effective angle of attack. An empirical trailing-edge separation correction is also applied to the transient force results. The airfoil body is undergoing large amplitude sinusoidal heaving and pitching motions at reduced frequencies of k = fc/U_∞ = 0.06,0.10, and 0.14. The airfoil body pitches about the mid-chord, and the heaving and pitching amplitudes of the airfoil are h₀ = 0.5c and Θ₀ = 70°, respectively, illustrating results for conditions near peak efficiency for energy harvesting. The leading-edge segment undergoes a relative sinusoidal motion with local angle amplitude of Θ_0,LE = 5° − 20L, and phase shift of φ_LE = 0−3π/2 relative to the pitching motion. Compared to a rigid airfoil, it is shown that the flexible structure of the airfoil enhances power efficiency by increasing the peak of lift force over the cycle and altering the timing of the local peak and the heaving velocity favorably. This improvement is a consequence of the change in the leading-edge vortex (LEV) inception and detachment times and the LEV growth rate of the circulation. The results show that the leading-edge motions with phase shift of π and 3π/2 provide better energy harvesting performance. Furthermore, higher leading-edge motion amplitudes favorably affect the leading-edge separation initiation and increase the power output, especially at higher reduced frequencies.