Final answer:
King Arthur did not appear in Old English literature; his legend is part of Middle English texts, with the Arthurian legends popularized after the Old English period, starting around 1150 A.D.
Step-by-step explanation:
No, King Arthur did not make his earliest appearances in Old English literature. The legend of King Arthur and the characters associated with his stories, such as Sir Lancelot and Camelot, are most prominently noted in Middle English texts, not Old English.
The Old English period, which lasted from 450 to 1066 A.D., is best known for such works as Beowulf, and while it was a rich time for Anglo-Saxon literature, the Arthurian legends as we know them today were not a part of this corpus.
Instead, stories of King Arthur were popularized later during the Middle English period, beginning around 1150 A.D., which followed the Norman Conquest of England in 1066. The most famous of these Middle English works is likely Geoffrey Chaucer's The Canterbury Tales, although Arthurian elements appear in various other Middle English romances and chronicles as well.