The use of different condensing and phosphorylating agents in conjunction with oxygen-nucleophilic catalysts, such as 4-substituted derivatives of pyridine N-oxide and quinoline N-oxide, leads to a dramatic increase of the rate of the phosphotriester bond formation and minimizes the amount of by-products caused by the modification of heterocyclic bases. The application of these catalysts to the solid-phase oligonucleotide synthesis allows to reduce the time needed for the performance of one elongation cycle on a polymer support to 10 min.