Two novel polynorbornenes 11 and 12 functionalized with electronically active conjugated oligomer units in the side chain were synthesized by the ring-opening metathesis polymerization (ROMP) method. Both polymers showed good optical characteristics, thermal stability, film-forming properties, and interesting electrochemical properties. The photophysical and redox behaviors of 11 and 12 are markedly different due to variation in the structure of the pendant oligomers. Polymer 11 with phenyl end-capped oligothiophene co-oligomer in the side chain showed much higher stability toward electrochemical oxidation than 12 with a sexithiophene in the side chain. This was demonstrated by in-situ study of the changes in absorption spectra of the polymer films while varying the potential in electrochemical experiments. During the p-doping process, polymer 11 exhibited highly reversible changes in its absorption peaks when monitored at 430 and 650 nm, and the p-doping/dedoping processes can be repeated many cycles. In sharp contrast, polymer 12 was shown to be electrochemically unstable under the same conditions. Single-layer photovoltaic cells have been fabricated with 11 or 12 as the active organic layer, and their relative performances were compared. These single-layer devices showed relatively large open-circuit voltage and moderate short-circuit current. In addition, the solar cell fabricated from 11 showed better device stability under ambient conditions than that from 12, which can be attributed to the higher stability of phenyl end-capped oligothiophene co-oligomer compared to that of the sexithiophene.