In our efforts to construct new metal–organic frameworks (MOFs) with open frameworks, three new metal oxalates, a 2D layered zinc oxalate [Co(en)₃][Zn₄(ox)₇]_0.5·5H₂O (1), a 2D saw-tooth layered manganese oxalate [Co(dien)₂][Mn₄(ox)₇]_0.5·6H₂O (2), and 3D open-framework cadmium oxalate [Co(dien)₂][Cd₄(H₂O)₂(ox)₇]_0.5·4.5H₂O (3), have been synthesized under hydrothermal conditions by using Co(en)₃Cl₃ or Co(dien)₂Cl₃ as template. Compared to the previously reported oxalate compounds that possess only medium sized 12-ring channels, the three open-framework oxalates exhibit larger channels, 16- and 20-ring channels. Compound 1 is a layered structure with large elliptical 20-ring channels linked by ten ZnO₆ and ten oxalates. Compound 2 exhibits a 2D saw-tooth sheet structure with 20-membered corrugated rings, and also presents an elliptical 16-ring aperture along [101] direction. Interestingly, compound 3 presents a 3D open framework, and Cd polyhedra and oxalate units alternate to form a 2D channel system that contains 12- and 20-ring channels, and it exhibits an unprecedented 3,4-connected mbc topology. Pairs of Co(III) complexes with opposite chirality are encapsulated inside the large channels as template. Our results suggest that using metal complexes as templates should be an effective strategy to construct MOFs.