Class O Pelagic planets range in age from 3 to 10 billion years old. Their diameters range in size from 10,000 to 15,000 km. They are always located in the ecozone region of a star's solar system. Class O worlds have an extreme abundance of water covering more than 80% of their surface. They have Class M-like atmospheres, with oxygen/nitrogen and other trace elements. There is almost always life, but almost all of it is aquatic plant and animal life, with little land mass to evolve surface creatures. Sentient races evolving on such worlds are likely to be aquatic in nature. (Star Trek: Star Charts)
Class O planets[]
External link[]
Class O planet article at Memory Beta, the non-canon Star Trek wiki.