PS, I think old English (being the older Celtic language) did not include the words crab or fish. Middle English (being old German, via the Saxons) did, though.
Old English was never a Celtic language
. I think you must look to Cornish and Welsh for Celtic etymologies, but I'm a student of Germanic, not Celtic, history so I don't know how applicable they are.
Depending on who you talk to, Old English was either an immediate development from Anglo-Saxon
or refers to the Anglo-Saxon language itself; and my dictionary of Anglo-Saxon includes "fisc" and "crabbe" for fish and crab.
Middle English is actually less of a Germanic language than Old English, because it includes the huge influx of Norman and French following the conquest.