Old English (Anglo-Saxon) is the early form of English which King Alfred spoke and in which Beowulf is written. As Professor of Anglo-Saxon, Tolkien taught this language, and as a writer he used its literature to inspire his fiction, but privately he saw himself as heir to the Old English of Mercia (the modern-day Midlands where he grew up), and he made this the language of the Riders of Rohan in The Lord of the Rings.
This lecture is from the lecture series, convened by Stuart Lee, presenting five Oxford academics who examine the medieval languages that J.R.R. Tolkien studied and taught. Each lecture consists of a short introduction to a language and its literature.