J.R.R. Tolkien studied Old English professionally while working on the Oxford English Dictionary.
After serving in the First World War, Tolkien joined the staff working on the dictionary in 1919. He worked mainly on words beginning with the letter W, examining their historical forms and relationships. His training in philology made him particularly suited to tracing how English vocabulary developed.
Old English, also called Anglo-Saxon, was the language used in England before the Norman Conquest. Tolkien later taught Old English at the University of Leeds and Oxford, and its vocabulary and literature strongly influenced his invented languages and stories. Words and names in his fiction often draw on Germanic language traditions.
Tolkien also studied Old Norse and other languages, so those subjects can be plausible alternatives. The question specifically concerns his professional dictionary work, where Old English was central to his contribution.