Oak Mountain State Park charges a fee to enter. Fees are $5/adult, $2/child, and $2 for seniors. You can also purchase a park specific annual pass for $180.
There are some spots along the trail that have gotten cluttered with new tree growth, but you can make your way through without too much frustration. The views are nice, too. You can park at the gate on John Findley Drive or at the campground parking area.