Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Qur'an was revealed in the Arabic language. It is known, in the Arabic language, that the plural personal pronoun can be used to refer to more that one (like in all other languages) or to denote the greatness of a single person. This is the imperial use of the personal pronoun.
In this context, the plural pronoun is used to denote the greatness and sublime nature of Allah. He (Allah) is indeed The One, The Single, The Glorious, The Great, High above everything else.
So, this does not contradict the Oneness of Allah.
Allah knows best.