This invention relates to olefin metathesis catalysts and methods for controlling olefin metathesis reactions. More particularly, the present invention relates to methods and compositions for catalyzing and controlling ring opening metathesis polymerization (ROMP) reactions and the manufacture of polymer articles via ROMP. This invention also relates to olefin metathesis catalyst compositions comprising at least two metal carbene olefin metathesis catalysts, wherein the at least two metal carbene olefin metathesis catalysts are structurally different, are chemically different, are distinct compounds, are not isomers, are not structural isomers, are not diastereoisomers, are not stereoisomers, are not enantiomers, or are not cis/trans isomers of each other, or any combinations thereof. This invention also relates to a ROMP composition comprising a resin composition comprising at least one cyclic olefin, and an olefin metathesis catalyst composition comprising at least two metal carbene olefin metathesis catalysts, wherein the at least two metal carbene olefin metathesis catalysts are structurally different, are chemically different, are distinct compounds, are not isomers, are not structural isomers, are not diastereoisomers, are not stereoisomers, are not enantiomers, or are not cis/trans isomers of each other, or any combinations thereof. This invention also relates to a method of making an article comprising combining an olefin metathesis catalyst composition comprising at least two metal carbene olefin metathesis catalysts with a resin composition comprising at least one cyclic olefin, thereby forming a ROMP composition, and subjecting the ROMP composition to conditions effective to polymerize the ROMP composition, wherein the at least two metal carbene olefin metathesis catalysts are structurally different, are chemically different, are distinct compounds, are not isomers, are not structural isomers, are not diastereoisomers, are not stereoisomers, are not enantiomers, or are not cis/trans isomers of each other, or any combinations thereof. Polymer products produced via the metathesis reactions of the invention may be utilized for a wide range of materials and composite applications. The invention has utility in the fields of catalysis, organic synthesis, and polymer and materials chemistry and manufacture.