The dark oak tree is a variant of the oak tree with a thicker trunk and a darker shade. They are found in the dark forest biome. Their trunks are at least 2x2 thick. To grow your own dark oak tree you will need a dark oak sapling that can be sourced from the leaves of a dark oak tree.
A dark oak tree (via screenshot)
The dark oak trees are naturally generated in the dark jungle biomes. They will always generate with a block of dirt underneath their trunk even if the tree is generated in water or partly in the air. They almost always will have irregular logs that are connected to the main trunk, these trees represent large branches.
A dark jungle (via screenshot)
The following items can drop from a dark oak tree :
You can grow your own dark oak trees with the help of dark oak saplings. To do so you will need to arrange 4 saplings in a 2x2 formation. They will not grow if the sapling is planted individually. They require a 3×3 column of unobstructed space at least 7 blocks above the sapling to grow (8 blocks including the sapling itself). This column is centered on the northwestern sapling. Additionally, dark oak trees require 5×5 layers without obstruction for the top 3 layers of their final height. No horizontal clearance is needed at the base of the tree (saplings planted in a 2×2 hole 1 block deep still grow).
Dark Oak trees are a variant of regular oak trees that have a darker brown shade than regular oak trees. These trees don't have anything special to offer other than the cosmetic differences.
There is a 1x1 variant of this tree but you cannot grow them as they are only generated in woodland mansions.
A. Since dark oak trees are only found in dark oak forests, they are rarer than your regular oak and spruce trees.
A. You will need four individual saplings placed in a 2x2 grid for a dark oak tree to grow.
A. A single dark oak sapling will not grow on its own, regardless of how long you wait or how much bone meal you use on it.