Here is my take on it. You will have to add your products to the cart. Because you are doing that you will most likely want to remove them from where they are on your site now. Since you are already doing that, you will probably remove the PayPal button.
I suppose, with modification of the cart, you might be able to bypass doing it that way but I am thinking it might be more work to do it than it would be to just use the cart as it is set up by default.
I have not used OSC in some time and there are other members here that do use it often so I will defer to one of them for confirmation.