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We present a new tool, called "OASIS" (Optimized Astrophysical Simulator for Imaging Systems), whose aim is to generate synthetic calibrated images of solar system bodies. OASIS has been developed to support the operations and the scientific interpretation of visible images acquired by the OSIRIS visible camera aboard the Rosetta spacecraft, but it can be used to create synthetic images taken by the visible imaging system of any spacecraft. OASIS allows takes as input the shape model of the object, in the form of triangular facets defining its surface, geometric parameters describing the position and orientation of the objects included in the scene and of the observer, and instrumental parameters describing the geometric and radiometric properties of the camera. The rendering of the object is performed in several steps which involve: (i) sorting the triangular facets in planes perpendicular to the direction of the light source and to the direction of the line-of-sight, (ii) tracing rays from a given facet to the light source and to the observer to check if it is illuminated and in view from the observer, (iii) calculating the intersection between the projected coordinates of the facets and the pixels of the image, and finally (iv) radiometrically calibrating the images. The pixels of the final image contain the expected signal from the object in digital numbers (DN). We show in the article examples of synthetic images of the asteroid (2867) Steins created with OASIS, both for the preparation of the flyby and for the scientific interpretation of the acquired images later on.