There are many Duas against the oppressors in Quran and Hadeeth of the Prophet (SAWA) and Ahlul Bayt (AS).
Quranic Dua : Verse 10 in Sura 54 (RABBI INNI MAGHLOOBUN FANTASIR)
فدعا ربه أني مغلوب فانتصر
It is also very useful to recite Ayatul Kursi 7 times for protection from the oppressors.
It is mentioned in authentic sources that reciting Dua Jawshan Al Sagheer is very useful for protection from the oppressors and any harm . Imam Musa Al-Kadhim (AS) recited Jawshan Al-Sagheer when the tyrant Abbasid ruler threatened to kill him, and the tyrant ruler dies in three days before even touching the Imam.
Istighfaar repeatedly is very useful also.