Death Valley in California is 86 m below sea level. Will the partial pressure of oxygen in Death Valley be the same, lower or higher than the partial pressure of oxygen at sea level?

It is not lower or higher. my best guess would be impossible to tell without additional information due to the fact that they only say partial pressure instead of the actual amount of pressure

Air pressure generally decreases (gets "thinner") with increasing altitude. Therefore, it generally increases with decreasing altitude. Since Death Valley is below sea level, its altitude is lower than sea level and the partial pressure of oxygen in Death Valley is therefore higher.