A solid support and two phosphoramidites exhibiting a phthalimide-oxy group were synthesized. First, after treatment with hydrazine, the resulting 5- and 3-oxyamine oligonucleotides were conjugated with aldehyde derivatives by oxime ligation. Second, oligonucleotides exhibiting an oxyamine at each end were circularized by means of different dialdehydes. Cyclic oligonucleotides of different lengths (9 to 31-mer) were rapidly obtained without the need of a template. Finally, a bicyclic oligonucleotide was synthesized starting form an oligonucleotide bearing three oxyamines, which reacted with a trialdehyde to form three oxime ligations.