Dryness & Dehydration

Dry skin generally refers to skin that is lacking in oil.
Dehydrated skin is characterised by lack of moisture (water) in the Stratum Corneum.
This explains why even oily skin can experience dehydration.
As mentioned, dehydration is a lack of water, not oil.
This means sebaceous oil activity can still be normal or even overactive in dehydrated skin.