When you first walk into the room where you are looking for Oka's chest, STOP, and look at the banners that appear on the wall for a short period of time. I think there are 3 of them that appear. Use the clues on the banners to light the objects that match them on the floor of the room. After you do that, Oka's chest will appear in the room. Hope this helps.
(If the banners fade away before you get to look at all of them, just back out of the room and come back in again. The banners will re-appear.)