6-O-7-N-Bis(diphenylcarbamoyl)-2-N-phenoxyacetyl-5'-O-dimethoxytrityl-2'-O-{[(triisopropyl- silyl)oxy]methyl}-8-oxoguanosine-3'-yl-β-cyanoethyl-N,N-diisopropylphosphoramidite (5) was synthesized as a new phosphoramidite precursor unit for the synthesis of RNA. Compound 5 was successfully incorporated into the middle of the RNA sequences, and the synthesized RNAs were identified by MALDI-TOF mass measurements. Their properties were evaluated for formation of the RNA duplex and RNA/DNA heteroduplex. ORNs 1 and 4 containing 8-oxo-G can form base pairs with rC or dC in an anti conformation, while it can also interact with rA or dA in a syn conformation in the RNA duplex or RNA/DNA heteroduplex. The described synthetic method is therefore a useful procedure for the synthesis of ORN containing 8-oxo-G and contributes to the study of 8-oxo-G in RNA.