Oppressed can speak evil (against oppressor)

Allâh does not like that the evil should be uttered in public except by him who has been wronged. And Allâh is Ever All¬Hearer, All¬Knower.

An-Nisa 148

Comment
Even in doing so,the oppressed must not transgress limits and if the oppressor makes a false allegation against him (particularly sexual) the oppressed should refrain from doing so as counter act.