The smart assembly of Co-containing polyoxometalates (POMs) and Co-based zeolitic imidazole frameworks (ZIF-67) is a promising tool to produce active electrocatalysts for the oxygen evolution reaction (OER). The encapsulation of SiW 11 Co inside ZIF-67 cavities via ' in situ ' synthesis produced two novel POM@ZIF-67 nanocomposites with low and high POM loadings-SiW 11 Co@ZIF-67 and SiW 11 Coh@ZIF-67, respectively. Other materials involving the analogue tri-substituted silicotungstate, SiW 9 Co 3 @ZIF-67 and SiW 9 Co 3 h@ZIF-67, have been prepared for comparison reasons. Regardless of the POM substitution degree, we demonstrated that the bidirectional synergistic POM-ZIF-67 interaction found for the already reported material, SiW 9 Co 3 @ZIF-67, is applicable to the other first-time prepared nanocomposites. Remarkably, this synergy is radically enhanced in the case of the two high POM loading samples, with these nanostructured materials being very active OER electrocatalysts, exhibiting 150-160 mV decreased overpotentials and much faster kinetics (∼35% decreased Tafel slopes) in relation to pristine ZIF-67. Moreover, they reveal intrinsic OER activities higher than those developed by the expensive commercial references (RuO 2 and IrO 2 powders) especially in the case of SiW 9 Co 3 h@ZIF-67. This important electrocatalytic improvement can be related to the higher POM loadings achieved, as well as to the formation of an open defect-enriched phase wrapping the 'standard' ZIF-67 framework in these nanomaterials. Increasing the POM loadings in POM@ZIF-67 nanocomposites prepared in situ results in extremely modified framework electrocatalysts with highly enhanced OER performances.