Entanglement generated by Spontaneous Parametric Down Conversion (SPDC) involves multiple, often mutually correlated degrees of freedom. These degrees of freedom are often treated independently, overlooking the intrinsic correlation between them. We focus on the spatial-spectral correlations that, if left uncontrolled, introduce distinguishability and reduce coherence, undermining applications such as high-dimensional OAM encoding. We analyze the spatio-spectral structure of the biphoton and identify conditions enabling a strong reduction of such correlations. We then quantify how spatial-spectral coupling degrades OAM spatial purity, mapping high-purity regions as functions of OAM order, crystal length, and pump/collection waists. The resulting design parameter regime enables engineering bright, high-purity OAM-entangled sources, reducing the need for additional loss-introducing filtering and therefore supporting scalable high-dimensional photonic quantum technologies.