Your oligonucleotide will be shipped as a 5' disulfide with the form HO-C6-S-S-C6-oligo. To reduce it to a free thiol, incubate with 100 mM DTT and 0.2 M phosphate buffer, pH 8.5, at room temperature, for 30 minutes. The product can then be purified with a G-25 column.