The categories of oppression are three:
1-The oppression of human towards the right of Allaah: such as shirk (associating partners with Allaah) and it is the most severe kind of oppression.
2-The oppression of human towards himself: such as when he burdens himself what he can not bear.
3-The oppression of human towards other than himself: such as killing the soul unlawfully and this is the most severe oppression in the right of human.
Extracted from Al-Fawaa’idul-Muntaqaah min sharhi kitaabit-Tawheed lish-shaykh Muhamamd bin Saalih Al-‘Uthaimeen page 8) by Isma’eel bin Mirshood Ar-Rumaih