Oily skin produces more sebum than average, which makes the skin look greasy.
To treat oily skin, you shouldn’t ever excessively wash your face. You might want to keep washing your face throughout the day since it feels greasy. Thoroughly cleansing your face is very important (double-cleanse!), yet washing your face too often can overly strip your skin, causing the skin to think it should produce even more oil. This is also why you shouldn’t leave your skin dry, skipping moisturisers - you’ll just even up with even more oil production! Rather, use oil-blotting papers throughout the day and use an oil-free moisturiser after cleansing.