There are a few Fragments to get and a Paradox ending to view in Oerba. The first Fragment is the Giant Egg Fragment, which you should have picked up on your first visit to this area. If not, it’s to the south of the point where you first land in Oerba. It may be hard to see because of the harsh glare coming off of the sand, but searching through the area should soon yield results.
The second Fragment is the Graviton Core Gamma, which again, you had to pick up to advance the story and should already have. If not, see the Guide on the Graviton Cores for instructions on how to find it.
The most significant extra in Oerba, then, is the Paradox Ending. You’ll get another Fragment for viewing this ending, so be sure to do this if you’re seeking out all of the game’s Fragments. You’ll have to face a boss battle with Caius in order to obtain this, and he will be stronger here than he was in the main storyline, so watch out. Make sure your party is ready; if you’ve beaten the game, you should be strong enough to beat him too. As in Yaschas Massif, you’ll have to enter the Pause menu and use the Paradox Scope on the Gate to Oerba along with the Encounters Gate Seal in order to close it, thus resetting story progress for this area. Jump through the various hoops required for the story until you are ready to fight Caius at the end of the area.
Boss Battle: Caius Ballad
As he did in the final encounter, Caius will use the Body and Soul ability to buff and heal himself. You should use Saboteurs to negate some of these effects and reduce his stats. If you allow Caius to buff himself, he will attack you until you have to heal, and then while you’re distracted with healing yourself his health will regenerate. Weaker post-game parties may encounter an issue where Caius is too weak to kill them, but they in return are too weak to kill Caius. In this circumstance, you should spam Saboteur abilities to debuff Caius while spamming Synergist abilities to buff yourself. When you stagger Caius, switch to the Cerberus or Relentless Assault Paradigms to deal heavy damage and bring him down.
Upon beating Caius, enter the Oerba (???AF) area to receive the Transcript: Vanille’s Truth Fragment and 10,000 experience points.