Module Isomorphisms of Maximal Orders

Question

I have been reading the paper Steinitz classes of central simple algebras and they make the following claim, just above Corollary 3.2:
Let $K$ be a number field with ring of integers $\mathcal{O}_K$. Let $\mathcal{A}$ be a central simple algebra over $K$ and suppose $\mathcal{O}_1$ and $\mathcal{O}_2$ are maximal orders of $\mathcal{A}$. Then $\mathcal{O}_1$ and $\mathcal{O}_2$ are isomorphic as $\mathcal{O}_K$-modules.
Could someone provide a proof of this fact?