We report a systematic study of small hydroxyl clusters on Cu(110), with scanning tunneling microscopy experiments and density functional theory calculations. Hydroxyl clusters are stabilized via hydrogen bonding, and their stability depends on the number of constituent hydroxyl groups. We reveal that there is a competition between hydroxyl groups in accepting hydrogen bonds, resulting in weak hydrogen bonding in odd-number clusters and stable dimer unit(s) in even-number clusters, as the hydroxyl on a metal surface is singly charged and has ability to form a strong hydrogen bond. Similar trends were found for other metal surfaces, suggesting a general feature of adsorbed hydroxyl groups.