Oh or OH or Oh! may refer to:

• OH-, the prefix for aircraft registered in Finland
• OH, the state of Ohio
• OH, the IATA abbreviation for the airline Comair
• OH (chemistry), hydroxyl functional group
• OH^- (chemistry), hydroxide
• ^•OH (chemistry), hydroxyl radical
• OH (chemistry), the functional alcohol group in a molecule, e.g. CH₃CH₂OH
• OH, in the Unified Soil Classification System, the designation for organic-rich clay or silt of high plasticity
• OH, a prefix of hominid fossils found at Olduvai Gorge, e.g. OH 13 ("Cindy")
• OH (military), the prefix for observation helicopter
• o.h. (architecture) overall height
• OH, text speak for "other half", i.e. partner or spouse

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

A hydroxyl is a chemical functional group containing an oxygen atom connected by a covalent bond to a hydrogen atom, a pairing that can be simply understood as a substructure of the water molecule. When it appears, it imparts to chemical structures some of the reactive and interactive properties of the -OH of water (ionizability, hydrogen bonding, etc.). The neutral form of the hydroxyl group is a hydroxyl radical. The anion form, (OH^-) is called the hydroxide anion; it bears a single negative charge largely residing on the more electronegative oxygen.

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