Considered all the oppressions - עשקים ashukim signifies any kind of injury which a man can receive in his person, his property, or his good fame.
On the side of their oppressors there was power - And, therefore, neither protection nor comfort for the oppressed.
So I returned, and considered - Rather, And I returned and saw. He turns to look upon other phenomena, and to test his previous conclusion by them.
Oppressed - See the introduction to Ecclesiastes.