End the game and goto theater mode, fast forward to the clip where you are at the barrier. Once there, you need to detach the camera. While pressing Y to detach, keep holding the RB to float upwards. You have to change the angle when detaching.

When you detach on the right angle, you will be able to float above the barrier. But don’t go to much, just enough to know you passed the barrier.

Now stay still and fast forward till the part where you die as the Oracle, and pause when its in the process of blowing up.

Now you can float up and completely out of the map and roam anywhere. You will not spawn back to the oracle, so this method is allows unrestricted movements outside the map.

View More Halo 3: ODST Glitches