Oxaloacetate (OAA) cannot cross the inner mitochondrial membrane.
The process of oxidative phosphorylation and the electron transport system (ETS) occur in the mitochondrion, whereas $\ce{NADH}$ generated by the reduction of $\ce{NAD+}$ in glycolysis is in the cytoplasm. The problem is that the inner mitochondrial membrane is not permeable to $\ce{NADH}$ , so a shuttle system is required for the transport of the reducing equivalents through the mitochondrial membrane. There are two of these, one of which is the ‘Malate–Aspertate’ shuttle.

In the process (see above) oxaloacetate (OAA) takes up the reducing equivalents from $\ce{NADH}$ to form malate in a reaction catalysed by malate dehydrogenase. The inner mitochondrial membrane is permeable to malate, which passes through carrier proteins (Malate-$\alpha$-ketogluterate transporter) into the mitochondrial matrix where it is converted back to OAA. As this happens the concentration of OAA decreases in the inter-membrane space, and as OAA cannot pass directly through the inner mitochondrial membrane it is converted into aspartate in the mitochondrial matrix by reacting with glutamate to produce $\alpha$-ketoglutarate and aspartate. The aspartate then travels to the inter-membrane space through specific carriers(Glutamate-aspartate Transporter). In the inter membrane space the aspartate combines with $\alpha$-ketoglutarate to form glutamate and OAA (the reverse of what happened in the mitochondrial matrix). Thus the concentration of OAA is maintained in the inter-membrane space, and the reaction continues.
Conclusion
OAA is converted into malate in the inter-membrane space. In the mitochondrial matrix malate is converted back to OAA, as illustrated in the illustration below.

Image source : Malate–Aspartate Shuttle, Wikipedia