Psalms 9:

I will thank and praise God for all his wonderful deeds.  My enemies turn back and perish before you because you have sat on the throne and give just judgment.  God rebukes the nations and make the wicked perish.  God is the stronghold of the oppressed and the troubled.  He has never forsaken those who seek him.  The wicked fall in the pits they dug and are trapped in the nets they hid. The needy and the poor shall not be forgotten, but the wicked and the nations that do not know God shall perish.

Psalms 10:

God are you hiding in times of trouble? The wicked oppresses the poor, boast of his desires, says that there is no God, and believes he will be unmovable for generations.  He gladly hunts for the helpless and seizes the poor.  He mocks God in his heart that God will neither do anything nor see his evil.  God, the helper of the fatherless, make the wicked account for their action till there is no wickedness left.  God is king forever and the fatherless as well as the oppressed that seek God will always get justice.  It seems that David (writer of Psalms) equates poor and oppressed with righteous or godly.  So the righteous are oppressed because they are few and therefore powerless in the power structure of the world in terms of money and law.