J.R.R. Tolkien’s Oxford academic specialty involving Old English was Anglo-Saxon, the historical language and culture of early medieval England.
Tolkien studied English language and literature and became a major philologist. At Oxford, he held the Rawlinson and Bosworth Professorship of Anglo-Saxon from 1925 to 1945. He later became the Merton Professor of English Language and Literature, a post he held from 1945 until 1959.
His scholarship included work on Beowulf, Middle English texts, and the history of Germanic languages. His famous lecture “Beowulf: The Monsters and the Critics,” delivered in 1936, helped reshape modern academic appreciation of the poem’s artistic qualities.
Anglo-Saxon is the older term commonly used in Tolkien’s academic title; modern scholars often prefer “Old English” for the language. The question concerns his Oxford specialty, not the invented languages of Middle-earth. Quenya and Sindarin were fictional languages created for his legendarium, whereas Anglo-Saxon was a real historical field of study.