The hydroxyl is the OH group of the organic compound, this is when the oxygen molecule is covalently bonded to the hydrogen atom. These can form hydrogen bonds with other molecules, due to the electronegativity between the oxygen and hydrogen molecule, with the oxygen having a slight negative charge, and hydrogen having a slight positive charge.
One example of a group of molecules with hydroxyl groups are alcohols, these form bonds with a carboxyl group to create an ester linkage. These are very important in biological complexes such as phosphodiester bonds found in the DNA double helix backbone.
Hydroxyl groups are also the differing factor between an RNA compound and a DNA compound. these are found on 2' carbon sugar on the pentose ring in RNA, and make it unstable. Hydroxyl groups are also in nucleotides to form hydrogen bonds to create the Watson-Crick DNA double helix structure. Forms hydrogen bonds.