Science

It seems there are oil eating bacteria that create a dead zone or oxygen deprived, but getting this info is now tough. Oil companies seem to crowd the discussion.
https://www.science.org/content/article/bacteria-are-gobbling-gulf-oil

Another concern has been that the oil-eating microbes could deplete oxygen within the plume. That's because the bacteria must extract oxygen from the water around them, which could spell disaster for local fisheries trying to get back on their feet. But neither study detected a dangerous drop in oxygen within the plume. "It would be very hard to establish a dead zone in this plume just because of microbial degradation," Camilli says.