Provided is a cycloolefin resin cured product having oxygen barrier properties prepared through bulk polymerization of a polymerizable composition comprising a cycloolefin monomer and a metathesis polymerization catalyst, wherein the cycloolefin monomer includes a norbornene-based monomer (a), the polymerizable composition comprises an antioxidant in an amount of 0.3% by mass or less, and the cycloolefin resin cured product has an oxygen permeability at 23° C. of less than 50 mL/m2·day·atm (100 μm).