Now a captain in the 1st Battalion, he commanded a company during the landing at X Beach on the Gallipoli peninsula on 25 April 1915 and acquitted himself well during the early stages of the fighting.

The commanding officer in the area, Brigadier General W. R. Marshall, eventually directed O'Sullivan to lead a party of Inniskilling and South Wales Borderers soldiers to capture the position which was achieved at dawn the following day.

The citation, published in the London Gazette on 1 September 1915, read as follows:[3] For most conspicuous bravery during operations south-west of Krithia on the Gallipoli Peninsula.

On the night of 1st–2nd July, 1915, when it was essential that a portion of a trench which had been lost should be regained, Captain O'Sullivan, although not belonging to the troops at this point volunteered to lead a party of bomb throwers to effect the recapture.

On the night of 18th–19th June, 1915, Captain O'Sullivan had saved a critical situation in the same locality by his personal gallantry and good leading.The wounds he received in the action of 1–2 July necessitated his evacuation to Egypt for medical treatment but he quickly recovered and returned to his unit on 11 August 1915.