O Assyrian; literally, Ho! Asshur. "Asshur" is the nation personified, and is here addressed as an individual. The transition from Isaiah 10:1-4 is abrupt, and may be taken to indicate an accidental juxtaposition of two entirely distinct prophecies.
Or Assyria may be supposed to have been in the prophet's thought, though not in his words, when he spoke of "prisoners" and "slain" in the first clause of Isaiah 10:4. The rod of mine anger (comp. Jeremiah 51:20, where it is said of Babylon, "Thou art my battle-axe and weapons of war; for with thee will I break in pieces the nations, and with thee will I destroy the kingdoms").
So Assyria was now the "rod" wherewith God chastised his enemies. The true "staff" in the hand of Assyria, wherewith she smote the peoples, was "God's indignation."