A month or so ago I was reading some early English history and the idea came to mind that Offa and Arthur sound so similar that perhaps Offa's name was just distorted into Arthur by word of mouth. The idea makes since as Offa's reign was around the same time as Arthur's supposedly occurred and he himself achieved much fame during his reign as a strong leader and conqueror. Could it be that through some misunderstandings, tales about King Arthur are really tales about King Offa?
Unless you are referring to Offa of Angel rather than the figure known as Offa of Mercia, i.e. the one that Offa's Dyke is named after, no.
Offa was an Anglo-Saxon king who lived during the latter half of the 8th century. King Arthur was a Briton (a Celtic people who inhabited the majority of Great Britain prior to the arrival of, and later alongside, the Anglo-Saxons - their heritage lives on in the Welsh and Cornish languages, among others) who allegedly lived during the late 5th and early 6th centuries.
This works much better in the modern English language than it would in Anglosaxon or in Welsh. Have "Arþúr" read to you in Icelandic and or "Arthwr" in Welsh over at www.ivona.com and and you will get the rough idea why "Arthur" couldn't've been distorted into "Offa" by any 9th century mouth.
Possible. Not really. The earliest earliest mentions of Arthur are, as Evan_Th mentions are largely contemporary with the Mercian Offas own time. So it would be unlikely to talk of a past mythological king when people who had actually met him were still around. Furthermore. the Historia Brittonum, written only a few decades after the Saxon king Offas death, speaks of Arthur fighting the Saxons. You wouldnt be writing about a man fighting off Saxons if you were referring to someone who was himself Saxon and not long dead. Offa of Angel, who I wasnt familiar with, seems to have no connection apart from the similarity in name.
I can see why Offa/Arthur might be seen as similar, but then you have a potential cause/effect thing. Perhaps Offa/Arthur of Mercias dad called him that BECAUSE it was similar to a mythological king. From an academic point of view you'd need a stronger connection than 'it sounds like'.