Charles Ellicott Commentary 2 Samuel 18:9

"And Absalom chanced to meet the servants of David. And Absalom was riding upon his mule, and the mule went under the thick boughs of a great oak, and his head caught hold of the oak, and he was taken up between heaven and earth; and the mule that was under him went on." — 2 Samuel 18:9 (ASV)

His head caught hold of the oak. —Absalom in his flight found himself among his enemies, and sought to escape into the denser parts of the forest. As he did so his head caught between the branches of a tree, his mule went from under him, and he hung there helpless. There is nothing said to support the common idea (which seems to have originated with Josephus), that he hung by his long hair, though this may undoubtedly have helped to entangle his head.