The invention herein describes a method to purify a target oligonucleotide using hydrophobic interaction chromatography (HIC). The method includes adding a salt to a mixture of the target oligonucleotide and product-related impurities, applying the diluted mixture, at a particular dynamic loading capacity, to the hydrophobic interaction chromatography resin (or hydrophobic adsorbent), washing the hydrophobic adsorbent with an aqueous salt solution, eluting the target oligonucleotide with a eluting solution, and collecting the eluent comprising the target oligonucleotide.