The present invention provides novel polyolefin compositions having variable hardness and/or toughness properties. The polyolefin compositions include: a polyolefin prepared by the metathesis of an olefin monomer using a ruthenium or osmium carbene catalyst; and one or more hardness and/or toughness modulators. The polyolefin is preferably poly(dicyclopentadiene), or poly-DCPD. Also provided are articles of manufacture, such as molded parts, produced from the polyolefin compositions. The disclosed compositions are useful in marine, recreational and sports products.