Biomes O' Plenty version: 1.6.4-1.1.2.51:

When using "Add Biomes To Default World"=true no ocean biomes are generating. You have to select the world type Biomes O' Plenty in order for the ocean biomes to generate.

The "Add Biomes To Default World" option is mainly for compatibility with existing worlds. You're meant to use the Biomes O' Plenty world type, as there are certain things that can only work with it, like ocean biomes.

Is the "Add Biomes To Default World" option needed if other mods add biomes to the overworld? I have a world that uses Biomes O' Plenty and The Mists of RioV right now. The Mists of RioV adds a few new biomes.

Biomes from other worlds will generate fine in the overworld with the BOP world type. By compatibility, I meant so people could have biomes in new chunks in older worlds without having to change the world type with an NBT editor.

Hmm. It does not appear to work. The Mists of Riov use something like "GameRegistry.addBiome(snowyMountains);" to add the snowyMountains biome to the Overworld. It works if worldtype is DEFAULT, but does not work if worldtype is BIOMESOP. How should a mod add a biome to the Overworld when the worldtype is BIOMESOP? Is it different if the worldtype is DEFAULT and when the worldtype is BIOMESOP?