Ocean planets do not merely have oceans like Earth do. They are entirely covered in extremely deep oceans and have no land or shallows at all.
A layer of phase II ice seals all geological activity and oxidizable metals from the water above, so even the small amounts of oxygen produced through photodessociation (UV radiation breaking water molecules into hydrogen and oxygen, and the hydrogen escapes to space while the oxygen stays in the atmosphere) is enough to make the atmosphere oxygen-rich.
There is no land to stop hurricanes, so there are lots of hurricanes. They mix the air with the water, oxygenizing the water. Life can exist, but only sparsely due to extreme scarcity of minerals. Sort of like the upper water layers of deep open ocean on Earth, just without the fertilizing Saharan dust.
One way they can form is if a icy world from the outer reaches of its solar system migrates closer to the star over time. Extra heat closer to the star then melts the ice, and an ocean planet forms. That type of ocean planet orbits relatively stable stars and have strong gravity, and thus will last long, especially if the star is small and consumes its own fusion fuel (hydrogen) slowly.
Another way for ocean planets to form is when the star becomes a red giant and melts small icy worlds in the outer reaches of its solar system, but the star is aging rapidly and the world has weak gravity, so that type of ocean planet (or more likely ocean moon) will not last long.