Oreburgh Mine is located at the south of Oreburgh City. There is a huge coal in the mine and you have find Roark here in order to battle him in the Gym. There are Pokémons in this mine, as well as Trainers if you talk to them.
Gym Leader Roark is found at the extreme south of Oreburgh Mine. If you talk to him, he will show you how to use Rock Smash. Afterwards, he will claim that he is the Gym Leader and went back to his Gym. Now head outside the mine towards the Gym and challenge the Gym Leader.