Compounds containing an oxygen atom, =O, doubly bonded to carbon or another element. The term thus embraces @A00208@, @C00852@, @K03386@, @S06118@, @A00266@ and @E02219@. Oxo used as an adjective (and thus separated by a space) modifying another class of compound, as in @O04376@, indicates the presence of an oxo substituent at any position. To indicate a double-bonded oxygen that is part of a ketonic structure, the term @K03382@ is sometimes used as a prefix, but such use has been abandoned by IUPAC for naming specific compounds. A traditional use of @K03382@ is for indicating @O04362@ of CHOH to C=O in a parent compound that contains OH groups, such as @C00820@, e.g. 3-ketoglucose.