Optical synthetic aperture (OSA) can greatly improve the spatial resolution of the optical system. However, due to the aperture dispersion and sparsity, the intermediate frequency (IF) modulation transfer function (MTF) of OSA imaging system is significantly lower than that of single aperture system. We first analyze the imaging method and principle of OSA imaging system, then discuss the reason and principle of the IF MTF reduction of OSA. Taking the filling factor as a clue, we propose the method of processing the IF MTF reduction with large filling factor and the IF MTF missing with small filling factor. For the IF MTF reduction, we use image restoration method to restore the IF information in the image. For the IF MTF missing, we fuse two images generated in two systems respectively to compensate the IF information. We analyze the feasibilities of the two methods and simulate the two cases in Zemax software. The results show that both methods are feasible.