Motivated by an observation of Namioka and Phelps on an approximation property of order unit spaces, we introduce the pp-tensor product and the mm-tensor product of two compact matrix convex sets. We define a new approximation property for operator systems, and give a characterization using the pp- and mm-tensor products in the spirit of Grothendieck. Thus, an operator system has the operator system approximation property if and only if it is (p,m)(p,m)-nuclear in a natural sense.