The difference is that the RM is the shortest, the Caprice is 1/2 inch longer, and the OCC is 1/2 longer than the Caprice. I do not know the overall length of each one. If you put an RM on a caprice or OCC, you will have a bare midriff (it ain't pretty). The Caprice tail on an OCC will give you the same effect. The aftermarket bumper covers are made for the Caprice lights, and the OCCs can be used with them as well. The Caprice and OCC lights can not be used on a stock RM, because the chrome trim will interfere with them. You may be able to use the OCCs on a Caprice, but the trim will probably have to be removed.