John Gill Commentary 2 Samuel 18:10

"And a certain man saw it, and told Joab, and said, Behold, I saw Absalom hanging in an oak." — 2 Samuel 18:10 (ASV)

And a certain man saw [it]
Saw him in the above posture, one of David's soldiers:

and told Joab, and said, behold, I saw Absalom hanged in an oak ;
caught by the neck in one, out of which he could not disengage himself, but there he hung, though alive.