Is the questioner considered to be an oppressor or oppressed after deciding to confront her friend about her flaws and her annoyance with her, which led to their separation, and what should she do for God to be pleased with her?

It is natural for problems to occur between colleagues at work, especially with the arrival of a new person who might cause anxiety to some. What you lack is understanding this colleague's personality and dealing with her in a suitable manner. What you did by confronting her with her mistakes is incorrect, as many people dislike confrontation, and the mistakes might only be from your perspective. It would have been better to gradually reduce your communication with her and not be forthright with her, especially since you know it would affect her.

Aisha (may Allah be pleased with her) reported: "A man asked permission to enter upon the Prophet (peace be upon him). He said, 'Permit him, for what a bad son of his tribe he is,' or 'What a bad man of his tribe he is.' When he entered, the Prophet spoke to him gently. Aisha said, 'I said, 'O Messenger of Allah, you said what you said about him, then you spoke to him gently?' He said, 'O Aisha, the worst of people in status with Allah on the Day of Resurrection is he whom people abandon or leave to avoid his obscenity.'" (Narrated by Al-Bukhari and Muslim).

It is advisable to maintain a superficial and amicable relationship with your colleague without delving deep, and to seek good companionship that brings you closer to Allah.