Access is along a gas pipeline on the south side of CR 141 just east of the bridge over the Withlacoochee River (i.e., here). We ended up parking on the shoulder of the highway. From the road, you follow the orange-blazed Florida Trail along a driveway to a gate. There is passage alongside the gate for hikers. This takes you on to a gas pipeline right-of-way which will take you in a southerly direction to the boundary of Suwannee River State Park. The gas line itself is desolate and depressingly straight, so concentrate on the scenery to each side--meadows and rich hardwood forest.
Crossing the park boundary, you're now on the loop. You can continue straight along the gas pipeline and do the loop clockwise, but we chose to turn right and do the loop counter-clockwise. The orange blazes lead you along the park's fenceline before making a sharp left onto an old road. Hikers follow the old road to the rim of a deep sinkhole, at which point the orange blazes lead you toward the Withlacoochee River and away from the road. Don't worry, you'll see the road again. It runs nearly to the southern tip of the peninsula, and the trail follows it on several stretches on the way south.
Past the sinkhole the trail takes you along the Withlacoochee River in the downstream direction. Depending on the water level in the river, you may hear the water rushing over some limerock shoals. After this first meeting, though, the Withlacoochee flows quietly between its steep limestone banks. Turtles sun themselves near the shores and hawks glide overhead. Not many other hikers shared the trail with us, but boats and kayaks went by on the river below, and families camped and fished on the opposite side of the Withlacoochee.
Very close to the south end of the loop you can look across the river and see the ruins of an old wall that was built to impound the waters of Suwannacoochee Spring, part of Ellaville, a vanished lumbermill town. Straight ahead you'll soon spot the picnic table and fire ring that mark the primitive campground at the southern tip of the peninsula. This is a good spot to stop, rest, and look at the confluence of the Suwannee River and the Withlacoochee River. Looking downstream, you can see three bridges that still exist in this area--the CSX rail bridge, the old US 90 bridge, and the current US 90 bridge. You can also look across the Suwannee at the more active portion of Suwannee River State Park (actually, you'll probably have already heard the noise from all that activity).
From the camping area, the trail heads back north (upstream) along the west bank of the Suwannee River. After about half a mile of following the Suwannee, the trail crosses the barren avenue of the gas pipeline. Huge signs warn boaters not to "anchor or dredge" where the pipeline crosses the river. Once you've walked across the pipeline, start keeping an eye out for the "Big Oak" for which the trail is named. Less than a quarter mile past the pipeline, the "Big Oak" will be on the left side of the trail. It's an old live oak that is significantly larger than the other big live oaks in the area.
Beyond the "Big Oak," the trail and the Suwannee River angle away from each other. There are no more river views, but plenty of woods, occasional sinkholes, and the possibility of wildlife sightings. Before completing the loop, you'll come to an intersection and the only sign (other than mileposts) on the loop. The orange-blazed Florida Trail goes right toward Key West, but you'll want to take the blue-blazed trail to the left that goes back toward your car. From the sign you follow the blue blazes back to the gas pipeline where you turn right. The pipeline takes you to the park boundary, where you complete the loop and rejoin the orange-blazed Florida Trail. Retrace your steps north along the pipeline back to CR 141.
The loop isn't absolutely flat, but it comes close. There were quite a few trees and branches down across the trail the day we hiked the loop (28 December 2008), but one work crew could change that.
More photos of the trail
Here are some links I used in planning our walk in the woods.
Grab and Go: Big Oak Trail
From the Florida Trail Association. You'll want the trail map.
Florida Hikes! Big Oak Trail
Sandra Friend's excellent take on the trail.