J.R.R. Tolkien’s Oxford academic field was philology.
Philology is the historical study of languages and texts, especially through changes in vocabulary, grammar, and written records. Tolkien trained in English language and literature and became an influential scholar of Germanic languages, Old English, Middle English, Old Norse, and related traditions.
At Oxford, Tolkien held academic posts including Rawlinson and Bosworth Professor of Anglo-Saxon and Merton Professor of English Language and Literature. His professional work included editing and translating medieval texts, while his language expertise helped him construct the languages and histories of Middle-earth.
Philology is not the same as linguistics in every modern academic sense, although the fields overlap. Tolkien’s approach was deeply historical and literary: he studied how words developed and how languages were preserved in texts and stories. His fiction grew partly from this scholarly interest, rather than being separate from his academic career.