A (UK) shortening of "Oh my actual god!" Sometimes used to indicate that you were so surprised that you could not finish the full phrase.

Popularized by Stephen Fry, on the TV game-show Q.I.
"Oh my actual… just been told I did my whole first two scenes with reading glasses round my neck. Shame-making …" Stephen Fry, Twitter status 432345626756800512
by Farthingale February 3, 2016
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