Porous monolithic materials have been prepared via ring‐opening metathesis polymerization from norborn‐2‐ene and a 7‐oxanorborn‐2‐ene‐based cross‐linker in the presence of porogenic solvents (i.e., 2‐propanol and toluene) and norborn‐2‐enephosphonate surface‐modified CaCO3 nanoparticles, using the 3rd‐generation Grubbs‐initiator RuCl2(Py)2(IMesH2)(CHPh). The experimental setup and the conditions chosen allowed for the manufacturing of polymeric monoliths characterized by a homogeneous distribution of the inorganic nanoparticles throughout the polymeric monolith. Depending on the nanoparticle content, the macropore diameters could be varied in the 30–120 µm regime. Noteworthy, the addition of nanoparticles did not affect the phase separation‐triggered formation of the monolithic matrix nor the meso‐ and microporosity as evidenced by N2‐adsorption experiments.