It is the way he moves.
People often notice that Hardy is graceful, smooth and precise in his movements; yet the aims are often futile - for example when he elegantly, with delicate finger-tip gestures, clears a tiny space for his eyes after his whole head and shoulders have been covered in oil or soot.
But gracefulness is not funny in itself, even when misapplied - a ballet dancer is not funny.
What makes Hardy funny is what happens just before the balletic movement.
Before he takes a step forward or moves his hand forward, Hardy makes a move back and away, and judders, quivers, oscillates with hesitation - before making the forward move.
And this happens again and again, with incredible seamless integration - so that he is continually hesitating then making graceful moves, uncertain oscillation then deft precision, all joined up in a way that has never been matched.
What is signalled is a desperate desire to do the right thing, which never succeeds, a tremendously considerate and compassionate uncertainty which always backfires.
After which he looks at the camera with infinite weariness, expressive of the shared knowledge that 'life's like that' - before some final overwhelming insult crashes down upon him.
Even Hardy's aggression seems without malice, and always backfires against him.
This is why Hardy is the best of all comedians, because he is as funny as anyone but more loveable than anyone who is that funny.
He utterly lacks the hard, masterful, technical manipulativeness of most great comics (e.g. his partner Laurel, Chaplin, Rowan Atkinson, Jim Carrey).
Hardy appears as a man of wholly natural goodness, in a world where he will always come off worst - despite those best intentions expressed in those hesitations.
Yet still he does what he does gracefully, precisely, delicately. No matter what happens, no matter how often disaster strikes, he will always be graceful.
That is why he is the funniest man who ever lived, and why we never tire of him; because Oliver Hardy was expressive of 'humour' in all its proper facets.