In the present study, the aerodynamic performance of the horizontal-axis wind turbine blades by considering the flap-wise oscillations are numerically investigated by using direct numerical simulations (DNS). The details of flow structure can be analysed and predicted by performing DNS over an oscillating blade by considering the realistic behaviour of the wind turbine blade structure with natural vibration frequencies. In this study, the impact of vibrations on the flow separation point, laminar separation bubble (LSB) and stall over NACA-4412 aerofoil are investigated utilising the high-fidelity spectral-hp element methodology. The Reynolds number and angle of attack were selected in the range of 50,000≤Re≤75,000 and 8^°≤AoA≤16^°. It is found that the blade vibrations have a noticeable impact on the aerodynamic performance and delay the stall occurrence, and the lift remains high even at higher AoAs, in comparison with the stationary blade. The size of the flow separation is reduced by the blade oscillation and the vibration also affects the separation point. Due to the harmonic oscillation of the blade, the pressure signals are periodic, and the pressure fluctuations are amplified by the oscillations, especially in the flow separation region. The time-averaged lift coefficient is increased by 255.3% by raising the angle of attack, from 0o to 12o at Re=75,000. Compared to Re = 50,000, the peak-to-peak amplitude for the angle of attack of 0 degree is higher, whereas that of 8 and 12 degrees are slightly lower at Re = 75,000.