The simultaneous conjugation of peptides or carbohydrates at the 3'- and 5'-end of oligodeoxyribonucleotides was achieved very efficiently through chemoselective oxime bond formation. The method employs bifunctionalised oligonucleotides in single step without the need of protection strategy, under mild acidic conditions. The conjugates were obtained in high yields by reacting an oxyamine containing reporter groups (peptide, mono- and disaccharide) with an oligonucleotide carrying an aldehyde at each extremity.