From April 1915 to August 1916 Harold Ozanne served on board HMS Warspite, one of the Royal Navy's most famous ships. She was launched in 1915 as the ultimate development of the modern battleship. Fast, heavily armoured and with huge fire power, throughout her 40 years in service she was constantly in the news and earned the most battle honours ever awarded to an individual ship in the Royal Navy.

After being rescued from the sea following the sinking of HMS Cressey in the North Sea, Harold Ozanne spent six months stationed in Plymouth. Then in April 1915 he was posted to the newly commissioned battleship HMS Warspite which had just completed its acceptance trials many of which were observed by Winston Churchill. Warspite began her slightly accident prone career by running aground in the River Forth and a few months later colliding with her sister ship HMS Barham causing herself considerable damage.

At the end of May 1916 Warspite took part in the Battle of Jutland and was hit several times by German shells killing 14 sailors. It also jammed her steering and caused her to sail off course and directly towards the German fleet, but also drawing fire away from some of her sister ships. The crew regained control of Warspite after sailing in two complete circles but due to her damaged condition she was ordered to return to Rosyth for repairs. She would be plagued with steering problems for the rest of her service life.

While on her journey home she was attacked by a German U-boat which fired three torpedoes, all of which missed their target. Warspite later attempted to ram a surfaced U-boat.

For his role aboard HMS Warspite at the Battle of Jutland, Harold Ozanne was awarded the Order of St Stanislas (2^nd Class with Swords) by the Russian Government for distinguished service.

In August 1916 Harold Ozanne left HMS Warspite to join the newly formed Royal Naval Division which was to join the British Army in France.