This is the first gym you come across in the whole Sinnoh region. You have to find Roark from the Oreburgh Mine because he is not there the first time you visit the city. He uses Rock-type Pokémon. The road towards the Gym Leader is straightforward. You can avoid the trainers there and battle the Gym Leader directly. However, you are encouraged to battle the trainers to earn experience points and cash, so that you are ready to battle with the Gym Leader.
It is recommended that you use a Grass-type or Water-type Pokémon to exploit Roark's Pokémon weakness. If you chose Chimchar at the beginning, it will be difficult to battle, so you have to catch a Pokémon that can exploit his Pokémon, such as Machop.
When you first visit Oreburgh City, a Youngster will talk to you and bring you to a Gym. Your rival is standing at the entrance, apparently he had defeated the Gym Leader. He said that the Gym Leader is not at the gym, he is currently at Oreburgh Mine mining rocks. Head to Oreburgh Mine to find him.
Tour the town and get free items! Talk to the worker near the mine to receive a free Super Potion. Then go to the house at the east to get Great Ball. Go to the house at the extreme left (near Oreburgh Gate) to get Dusk Ball.
If you have a Zubat(or a Geodude in Platinum) in your party, visit the second house next to Oreburgh Gate and show it to the plump man at the second floor. He will give you a Heal Ball.
You can access Eterna City directly at Route 207. But it is inaccessible without a bike. So you have to go back to Jubilife City and head north to make a detour to Eterna City. You will get bumped by your friend when you are heading to the Oreburgh Mine, he will explain for you and then run off to Eterna City.