Just weeks after being commissioned, Ocean was undertaking the warm water element of her first-of-class trials, when she was deployed on short notice to the coasts of Honduras and Nicaragua to provide humanitarian assistance in the aftermath of Hurricane Mitch. In early 1999, Ocean was scheduled to take part in an exercise in the Atlantic, but was diverted to the Mediterranean in readiness for possible deployment to Kosovo.
During 2000, Ocean supported Operation Palliser in Sierra Leone, joining Illustrious in aiding the suppression of rebel activity with her own EMF and providing support facilities for the Spearhead battalion ashore.
On 17 February 2002, a unit of Royal Marines from Ocean accidentally landed on the San Felipe beach in the Spanish town of La Linea instead of Gibraltar, causing a minor diplomatic incident as various media outlets labelled the mistake as an "invasion".
Ocean was part of a large Royal Navy task force deployed for Operation Telic, the UK contribution to the 2003 Iraq War, for which she was awarded a new battle honour "Al Faw 2003". In the helicopter assault role she was accompanied by Ark Royal.
HMS Ocean showing landing craft on davits and stern ramp deployed
In the summer of 2006, she was deployed as part of the task force involved in the Aurora exercises on the eastern seaboard of the United States.
In 2007, Ocean began her first long refit period. This was carried out by Devonport Management Limited at their Devonport Royal Dockyard facility and lasted around 12 months, during which period Ark Royal took over the LPH role. Ocean sailed from Plymouth on Wednesday 24 September 2008 to start sea trials, following this major period of maintenance and upgrading work. As part of that upgrade, a PyTEC pyrolysising waste recycling unit was fitted.
On 18 February 2009, Ocean sailed from Devonport as part of the Taurus 09 deployment under Commander UK Amphibious Task Group, Commodore Peter Hudson. She was joined on this deployment by the landing platform dock Bulwark, as Hudson's flagship, Type 23 Frigates Argyll and Somerset and four ships of the Royal Fleet Auxiliary. This exercise was filmed for the second series of the Channel 5 documentary Warship.
In June 2009, she took part in exercise Bersama Shield with Somerset and RFA Wave Ruler off the Malay Peninsula.
During the air travel disruption after the 2010 Eyjafjallajökull eruption, Prime Minister Gordon Brown assigned Ocean and other units to rescue stranded travellers and army personnel across the English Channel in Operation Cunningham.
In 2010, she was deployed on a multi-purpose deployment. This started with exercise Auriga on the eastern coast of the USA. She then moved to Brazil to conduct an exercise with the Brazilian marines, whilst there a defence co-operation agreement was signed on board. She then crossed the Atlantic heading for Nigeria to both participate in the Nigeria at 50 presidential fleet review and capacity building with the Nigerian navy as part of the African partnership programme. She returned to Devonport in November.
In April 2011, she was deployed as the follow on ship for the RN's Response Force Task Group (RFTG) COUGAR 11 deployment. During this deployment, she took part in Exercise Cypriot Lion
In May 2011, she was detached from the COUGAR 11 deployment of the Response Force Task Group and sent with embarked Apaches to aid operations in Libya along with the attack helicopters aboard the French amphibious assault ship Tonnerre. This marks the first time that Apache helicopters have been sent into action from a Royal Navy ship. Her initial complement of three Apaches was bolstered by a fourth soon after, and later a fifth. The deployment included a large medical team, a sign of the ship's flexibility.
On 4 May 2012, she moored at Greenwich to prepare for her role of providing logistics support, accommodation and a helicopter landing site during the London 2012 Olympic Games. From 24 to 28 May 2012, she visited Sunderland, her affiliated port, and made other port calls before returning to London on 13 July. After Olympic duty, Ocean returned to her home port of HMNB Devonport for a scheduled period of maintenance. The LPH role was provided by HMS Illustrious until 2014. On 22 July 2014, HMS Ocean took over the helicopter carrier role again, after its 15-month, 65 million Pound refit, replacing HMS Illustrious, which then returned to its home port Portsmouth for a final time, being decommissioning on 28 August 2014. As part of the Response Force Task Group COUGAR 14 deployment, Ocean has participated in exercises off Albania and France.