"Oxygen levels fell significantly in deep-sea areas of the Gulf of Mexico contaminated by oil plumes from the BP spill. But although researchers found a 20% decline in dissolved oxygen, the drop was not steep enough to create biological 'dead zones' that some scientists feared might form in the wake of the BP disaster." Those were the findings of a government study.
"Gulf Oil Spill: Oxygen Dropped Near Oil Plumes"
Source: LA Times, 09/08/2010