In the poetry of many nations, the interjection O! is a marker of poeticalness, a marker that contributes to the factors distinguishing poetry from colloquial speech. O! is treated not so much as an expression derived from the language in which a given poem was written (i.e., English, Italian, Polish, etc.) as a common lexeme within an international poetic language. In different countries, the interjection O! is understood in similar ways and does not require translation, even if the other parts of the poem are rendered in distinct languages. Despite the importance of the interjection in world literature, research into the semantics of O! has been limited in scope. The aim of this article is to trace the main stages of development that O! has undergone in European poetry from antiquity until the present day. The article initially discusses the semantic variants of the interjection in ancient Greek and Latin poetry. These derive from two functions of O!, functions that are described within the context of the Bakhtinian concepts of the addressee and superaddressee. Subsequently, the process in which the autonomy of this lexeme was shaped with regard to vernacular languages is considered. The examples illustrating this process have been taken from Bulgarian, English, French, German, Italian, Occitan, and Polish poetry.