how to substract two images using opencv

My vote of 1: Just asking for code makes most people assume you want to be given an answer. As we are all unpaid volunteers, it is unlikely you will get any real help until you show some effort.
Please re-phrase your question so that we know what you have tried and specifically what you are having problems with.
You should also tell us why upyou need OpenCV, as SAKryukov states, this is a just a matter of subtracting pixel-by-pixel.