Inorganic-organic hybrid structures belonging to the family of iron phosphite-oxalates have been prepared by employing hydrothermal methods. Their structures, determined by single-crystal X-ray diffraction, show a hierarchy within the family. While compounds I and II are low dimensional, III-VI have three-dimensional structures. Compound I has edge-shared ladders of iron phosphite with oxalate units hanging from the iron centers. Compound II has a layer structure with a honeycomb-like arrangement. The three-dimensional hybrid structures have the oxalate units connected in both in-plane and out-of-plane modes. A newly identified secondary building unit (SBU-7) and the oxalate units satisfying the valence and coordination requirements in the structure of V are novel and noteworthy structural features. Magnetic studies show that the dominant interactions between the iron centers are antiferromagnetic. Similar to other known hybrid structures, the phosphite-oxalate structures appear to show wide compositional and structural diversity.