I think the OP is referring to lower amounts of dissolved oxygen at greater depths.
https://en.m.wikipedia.org/wiki/Oxygen_minimum_zone
The dissolved oxygen at shallower depths will be much higher than at deeper depths. For one, it's closer to our gaseous atmosphere so oxygen enters water that way. The underwater plants also undergo photosynthesis and produce oxygen, but these plants are unable to survive at greater depths due to lack of sunlight.
Basically, at a certain depth there is no oxygen production. All the oxygen that's in deep water is a result of shallower water circulating down into deeper depths.
Now, when you factor in the quantity of oxygen-using organisms, the deep deep ocean may have more dissolved oxygen than at close-to-surface depths because even though there is nothing producing oxygen at these depths other than the natural churning of water, there are also vastly fewer organisms using that oxygen. There is a middle depth though where there is, again, no oxygen production, but a greater quantity of life, so the middle ocean will have the lowest amount of dissolved oxygen.