Heading South on I-35 exit William Cannon and head east until you dead end into McKinney Falls Pkwy go left and then take another left into the park. Pay at the booth and take your first right, go to the end and park. There's a trail that leads to a opening - walk across the limestone slab to the left. Once at the edge look for the oak tree to get down.
Day use fee is now $6 per person. When the front gate closes use the self pay station at the front gate. Not free after 5. There is a night patrol. The 11/30/13 flood has left a lot of debris. The map point shows Onion Trail not the bouldering area but the location description is accurate. There are usually maps at the self pay station that clearly mark the bouldering area but not the routes.