From ‘Salopia antiqua’ by Charles Hartshorne (1841) p79:
“It is called ‘Hen Dinas’ or the old city; and anciently Caer Ogyrfan, or Ogyrfan’s Castle, who was a hero contemporary with King Arthur.”
– Ogyfan was the father of Guinevere, of the King Arthur myths.
You can read more about the legends at ‘buzzpages’ buzz-pages.com/features/oswestry/Fest2000/OldOS.htm
which has excerpts from the ‘History of Oswestry’ by a Mr Watkins.