Oily skin and combination skin both have one thing in common: a recurring problem of sebum and shine. It is the location of oily areas that differs between oily skin and combination skin.
Oily skin has excess sebum all over the face with all the associated signs: a shiny nose, enlarged pores, a dull complexion. Sebum production is excessive in all areas of the body with sebaceous glands. This is an advantage since sebum has a mission to protect the skin, but at the cost of an aesthetic that is often difficult to live with!

Combination skin has excess sebum in the T zone. Areas of the face outside the T zone can be normal at best, dry and reactive at worst. An imbalance that makes choosing appropriate care very difficult, since it only needs water in certain places, water and fat in others. Combination skin is very unstable and changes depending on environmental factors.