An oligomer having, as the constitutional component thereof, (dihydrodicyclopentadienyl)-maleates or -fumarates of the general formula I: ##STR1## (I) wherein, R is H or ##STR2## and possessing a softening point in the range of from 50.degree. to 180.degree. C. and solubility in organic solvents. It is obtained by polymerizing the compound having the above general formula I under heating at a temperature of 220.degree. to 300.degree. C. in the absence of a catalyst.