Is it true the more you got oppressed, the more strong you are?

In some circumstances, yeah. It can force people to learn to stick it out in tough times and fight viciously for their rights. It can give people a sense of empathy for others in the same situation. On the flipside though, it can wear down people's self confidence and self esteem, drive them into a state of bitterness towards society in general, and otherwise break down their spirit.