Can Oxaloacetate cross the outer mitochondrial membrane?
In general the outer mitochondrial membrane is thought to be basically permeable (through porins) to small molecules such as OAA. As is typical in biology, the situation may actually be more complex -- see for example this paper. But I think the default assumption is that metabolites freely cross the mitochondrial outer membrane.

You might also ask whether either the mitochondrial MDH or the cytoplasmic MDH enzyme is likely to be found or be active inside the intermembrane space.