We show that in a locally compact complete $CAT(0)$ space satisfying positive angles property and a disintegration regularity for its canonical Hausdorff measure, there exists a unique optimal transport map that push-forwards a given absolutely continuous probability measure to another probability measure. In particular this holds for the Riemannian manifolds of non-positive sectional curvature and $CAT(0)$ Euclidean polyhedral complexes. Moveover we give a polar factorization result for Borel maps in $CAT(0)$ spaces in terms of optimal transport maps and measure preserving maps.