Many Ocula nodes rely on disparity maps to produce their output. Usually, disparity maps are created using a combination of the O_Solver and O_DisparityGenerator nodes.
However, if you have a CG scene with stereo camera information and z-depth map available, you can also use the O_DepthToDisparity node to generate the disparity map. Provided that the camera information and z-depth map are correct, this is both faster and more accurate than using the O_Solver and O_DisparityGenerator nodes.