At which university did J.R.R. Tolkien serve as a professor before returning to Oxford?

J.R.R. Tolkien served as a professor at the University of Leeds before returning to Oxford.

After working on the Oxford English Dictionary and teaching at the University of Leeds, Tolkien became Rawlinson and Bosworth Professor of Anglo-Saxon at Oxford in 1925. His Leeds appointment helped establish his academic career in the study of language and medieval literature.

At Leeds, Tolkien taught Old and Middle English and collaborated with E.V. Gordon on the 1925 edition of Sir Gawain and the Green Knight. His academic work and his fascination with language fed directly into the linguistic construction behind his fictional peoples.

The question is sometimes answered with Oxford because Tolkien is most strongly associated with that university. Oxford was his later and longer academic home, including posts at Pembroke College and Merton College, but Leeds came first.