Osterhaus continued in division command during the Vicksburg Campaign, fighting in the Battle of Champion Hill and at the Battle of Big Black River Bridge, where he was slightly wounded. Osterhaus's division made an unsuccessful first attack on the defenses of Vicksburg, the first act of the Siege of Vicksburg. His division helped cover the siege against intervention by the Confederate forces of Gen. Joseph E. Johnston, and he took part in Maj. Gen. William T. Sherman's advance on Jackson, Mississippi, that was designed to protect the rear of the Army of the Tennessee in its siege operations.