In which location would you expect the oxygen concentration to be the lowest?

The location that should have lowest oxygen concentration would be the tissue because it is the furthermost location.

Other location with lowest oxygen concentration should be in the right part of the heart. It is when all unoxygenated blood pooled before going to the lungs. But the left part of the heart will have highest oxygenated blood, so it's ambiguous.

High oxygen concentration should be found in the environment, lungs, and the blood.