OmegaCAM, the wide-field CCD camera for the VLT Survey Telescope, will have an on-line image analysis system, the design of which will be described here. Aim of the system is to provide continuous, real-time image aberration coefficients that can be used to correct the telescope primary mirror figure, and the secondary mirror position and attitude. Wavefront sensing is performed using a curvature-like system, which consists of two CCDs mounted out of focus (one in front, one behind focus) in the outer part of the focal plane. During observations short exposures of bright guide stars are continuously read out, the defocussed images are then decomposed into terms that correspond to wavefront aberrations. The optical design of the wavefront sensor, the image analysis algorithm and software, and system operations will be described.