Listed by the International Mountain Bike Association as an Epic Ride, Oak Mountain State park has a whopping 26 miles of trails from beginner trails to fast and technical downhills.
Oak Mountain State Park has some great mountain biking for all ages and all skill levels. Most trails at Oak Mountain are more oriented to the cross country rider, but you can find a couple of trails more geared towards downhill, gravity riders. However, expect to be cross country mountain biking.
The pin takes you to the South Trailhead of Oak Mountain State Park. This is where most of the mountain bike trails begin. There is plenty of parking and plenty of trails to begin on. There is a full map of the trails posted when you arrive at the South Trailhead to give you a better sense of where to go. Be aware that there is a $5 fee per person to get inside the park.
For the beginner mountain bike rider, a few trails that are great to begin on are Lake Trail, Family Trail, or Mr. Toad's. These trails offer some fun flowy sections and a scenic ride without being too technical.
For the advanced rider, trails such as Blood Rock or Jekyll and Hyde would be more suited for you. Both trails offer very technical rides with lots of rock and boulder obstacles. Being very familiar with Jekyll and Hyde, I would say that it is one of my favorite in the park. While the top half of Jekyll and Hyde can be very difficult, the last half of Jekyll and Hyde is incredible. Very flowy and fast.
For downhill riders, you have about 2 options. A very popular trail for downhill riding is Lightning. With wood features and big drops, this trail is super fast and has some good sized jumps but is very short at 0.3 miles. However, by taking the Red Road, you can access a newly developed trail called Thunder. Thunder is more than double the length of Lightning and connects to Lightning at the very end.
A detailed picture of the trails can be found here