Crosslinked polydicyclopentadiene polymer and copolymer are made by first forming a thermoplastic polymeric intermediate in a ring-opening metathesis polymerization (ROMP), and then crosslinking the intermediate in a melt-processing or solution processing step. The formation of the intermediate permits facile removal of residual monomer, which leads to a reduction in odor and improvement in physical properties. Crosslinking can be achieved using various crosslinking strategies, including further ROMP reactions, addition polymerization of residual double bonds, addition of a crosslinking agent or introduction of functional groups.