This paper describes the synthesis of polylactide-functionalized polyoxanorbornenes and their hydrolytic degradation behavior. Macromonomers with one or two exo-PLA chains as well with two endo,exo chains were prepared using tin(II) 2-ethylhexanoate as a catalyst in the presence of mono- or dialcohol derivatives of oxanorbornene. The well-characterized macromonomers were then subjected to ROMP by first, second, and modified second generation Grubbs ruthenium initiators. Investigation of these graft copolymers by SEC and NMR spectroscopy showed the presence of some uncapped PLA homopolymer, formed as a side product during the ROP of lactide. The degradation behavior showed that the presence of PLA homopolymer impurities in the graft copolymers significantly increases the rate of degradation of the final material. Therefore, a convenient procedure of graft copolymers purification to remove PLA homopolymer from the samples was developed. The degradation studies of graft copolymers with the same oxanorbornyl backbone length and different length of PLA grafts indicated that the degradation rate increased with increasing length of PLA grafts. The degradation behavior of material depends also on configuration of PLA side chains on polyoxanorbornene backbone chain. The fastest degradation was observed in the case of graft copolymers with one exo-PLA side chain.