Oligodeoxyribonucleotides were synthesized using H-phosphonate monomers without amino protection. The H-phosphonate monomers of deoxyadenosine, deoxycytidine, and deoxyguanosine bearing the free amino groups were synthesized in good yields by O-selective phosphonylation of the parent 5'-O-(dimethoxytrityl)deoxyribonucleosides. It was found that the amino groups of the nucleosides were not modified during internucleotidic bond formation where (benzotriazol-1-yloxy)carbonium and -phosphonium compounds were employed as condensing reagents. The most effective condensing reagent for rapid internucleotidic bond formation was found to be 2-(benzotriazol-1-yloxy)-1,1-dimethyl-2-hexafluorophosphate (BOMP). In the present H-phosphonate method, 2-(phenylsulfonyl)-3-(3-nitrophenyl)oxaziridine (PNO) was employed as a new oxidizing reagent for the oxidation of internucleotidic H-phosphonate linkages under anhydrous conditions in the presence of N,O-bis(trimethylsilyl)acetamide. The reaction mechanism for the O-selective condensation was investigated in detail by means of P-31 NMR spectroscopy. Unprecedented oxidation of the H-phosphonate monomers was observed during activation of the monomers with (benzotriazol-1-yloxy)phosphonium and -carbonium condensing reagents in the absence of the 5'-hydroxyl components. A mechanism for the O-selective condensation was proposed on the basis of ab initio molecular orbital calculations for the model compounds at the HF/6-31G* level.