Object language, for the language about which an analysis is given, as opposed to the language in which the analysis is formulated, is a borrowing of German Objektsprache. Since object language is almost always contrasted with metalanguage, and metalanguage is universally attributed to Tarski, I assumed until recently that Tarski was also responsible for object language. That turns out not to be the case. Objektsprache actually originates with Carnap (1934), p. 4.
For the language in which an analysis is framed, Carnap used Syntaxsprache ("syntax language"), which never caught on in the way object language and metalanguage did.
The earliest attestation of English object language is in Stebbing's (1935) review of Carnap (1934) and three other books by Carnap (Oxford English Dictionary); but the (1937) English translation of Carnap (1934) should probably be regarded as the main source through which object language entered the English logical and semantic vocabulary.
- Carnap, Rudolf (1934) Logische Syntax der Sprache. Vienna: Verlag von Julius Springer.
- Carnap, Rudolf (1937) The Logical Syntax of Language. London: Routledge and Kegan Paul.
- Stebbing, L. Susan (1935) Critical Notice of Logische Syntax der Sprache, Die Aufgabe der Wissenschaftslogik, Philosophy and Logical Syntax, and The Unity of Science by Rudolf Carnap. Mind 44.176.499–511.