From October–December 1914, she served as flagship of the squadron in the Persian Gulf supporting operations against Basra. During this period, she escorted an Indian troop convoy to Bahrain in mid-October; the convoy had departed India on 16 October, and "Ocean" met the convoy at sea three days later to take over escort duties. After the convoy arrived on 3 November, "Ocean" began making preparations for the attack on Basra. Numerous small craft were armed to enter the Shatt al-Arab, the river leading to Basra. The first objective was to silence the old fortress at Al-Faw. On 5 November, Britain declared war on the Ottoman Empire, and the next morning, "Ocean" began the bombardment of Al-Faw. A landing party of 600 men, some of whom came from "Ocean"s detachment of Royal Marines, stormed the fortress and captured it, having encountered no resistance.