Kirby and Berry begin the final storyline in OMAC 7 (Sept/Oct 75).
Large bodies of water are mysteriously vanishing around the world, leaving behind them dead sea life and plants. OMAC goes to investigate one of these regions.
He finds an unusual cube on what had once been the floor of the sea. He cannot lift it, and even with strength added by Brother Eye is barely able to move the extremely dense object. Examining it from afar, Brother Eye figures out what it is.
But we don't hear the explanation from Brother Eye, instead the story jumps over to the villain, Dr Skuba, and we watch how he uses an atomic powered machine to pull together all the water from a lake, compressing it all into one of these cubes. He has a larger cube, with which he plans to steal the Atlantic Ocean, and hold the world for ransom before he will return it. He appears to have two children working with him, though we will later discover these are also creations of his, using free atoms. Kirby just loved free atoms. They appear all over his writing.
OMAC goes in search of Dr Skuba, and thanks to Brother Eye doesn't have too much trouble tracking him down. But as he approaches, Dr Skuba has time to study OMAC, and learns that the hero is a transformed human.
Skuba uses his atomic weapon to remove the power that Brother Eye has endowed OMAC with, returning him to being just Buddy Blank. The way that OMAC has no memory of his days as Buddy, Buddy has no memory of ever being OMAC, and has no idea what he is doing lying on a rock.
The story concludes in the next issue.