Rotten Oak Log

Rotten oak logs are a variant of oak logs.

Natural Generation
Rotten oak logs are the main blocks of rotten swamp trees, along with vines.

From Block Loot
Rotten oak logs can be mined by hand, though axes can speed it up. It drops itself.

Crafting
4 rotten oak logs makes 3 rotten oak woods.

Lore
Rotten oak logs are oak logs that have started to decay. They are no longer alive, are usually saturated with water, and have all sorts of molds, mosses, and grubs (oh my!) living in them. They are so decayed that pretty much nothing can be done with them. If an attempt were made to modify them, such as splitting them for planks, they would just crumble and rot.

It is unlikely to find any other types of wood decayed like this (and nothing else is in the mod), simply because only oak can be found in conditions right to cause this amount and type of decay easily.