Thráin II is the father of Thorin Oakenshield in J.R.R. Tolkien’s legendarium.
Thráin II was a Dwarf of the House of Durin and became King of Durin’s Folk after the death of his father, Thror. He was driven from Erebor after the dragon Smaug seized the Lonely Mountain and its treasure. Tolkien’s histories describe Thráin’s later wandering and imprisonment by Sauron’s forces.
Thorin, Thráin’s son, eventually led the company that traveled with Bilbo Baggins to reclaim Erebor. Thorin’s title, Oakenshield, refers to the improvised shield he used during the Battle of Azanulbizar, an event that took place before The Hobbit.
Thror is therefore Thorin’s grandfather, not his father, while Dáin II Ironfoot is Thorin’s cousin and successor as King under the Mountain. These closely related Dwarven names are a frequent source of mix-ups.