Oh, OH or Oh! is an interjection, proclaiming surprise. It may refer to:

• OH, hydroxyl functional group
• OH^-, hydroxide
• ^•OH, hydroxyl radical
• OH, the functional alcohol group in a molecule

Hydroxide is a diatomic anion with chemical formula OH^-. It consists of an oxygen and hydrogen atom held together by a covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, a ligand, a nucleophile and a catalyst. The hydroxide ion forms salts, some of which dissociate in aqueous solution, liberating solvated hydroxide ions. Sodium hydroxide is a multi-million-ton per annum commodity chemical. A hydroxide attached to a strongly electropositive center may itself ionize, liberating a hydrogen cation (H⁺), making the parent compound an acid.

A hydroxyl or hydroxy group is a chemical functional group containing one oxygen atom connected by a covalent bonding to one hydrogen atom (-OH). It is sometimes called the alcohol functional group because when bonded to carbon in a molecule that otherwise contains only hydrogen and carbon the hydroxy (not hydroxyl) group defines the molecule as an alcohol, resulting in a name ending in -ol. A hydroxyl group bonded covalently to the carbon of a carbonyl group (C=O) produces a carboxyl group (C(O)OH) that is the defining group of a carboxylic acid. When the -OH group participates in an ionic bond, the [OH^-] anion is called the hydroxide ion. As a free radical, it is the hydroxyl radical.

"Oh!" is the lead single from Oh!, the second studio album by South Korean girl group Girls' Generation. The single was digitally released on January 25, 2010 and came to be a huge hit song in South Korea. The group used a cheerleader concept for the song's promotional activities. The song hit #1 on various charts, and won several times on the Korean music chart shows Music Bank and The Music Trend. The song ranked #4 on Gaon Chart Top 10 Digital Songs of the Year and it was also the second best selling single in that year, selling over 3.3 million copies. Music site Monkey3 named "Oh!" the best song in 2010.

"Oh!" was the 22nd single (and last official single before their break-up) of Pink Lady. The single was released on March 5, 1981.