The difference is Ocean, as well as the Invincibles, and the Colossus and Majestics, is they were designed using finite element analysis and as such has far less magin for error designed into them. She was designed to do a specific job for a specific period with no margin for major upgrades or life extensions, the only reason the Invincibles, and Type 23s for that matter, were able to serve for so long is because they didn't spend most of their service in the North Atlantic hunting soviet submarines as expected, but rather in more temperate climates showing the flag and the occasional warm war.