In his book Justice Nicholas Wolterstorff coins a great phrase to highlight the concerns in the Old Testament to care and seek justice for the weak and vulnerable. Wolterstorff calls these vulnerable groups "the quartet of the vulnerable": the poor, the foreigner residing within your borders, the orphan and the widow.
The quartet is mentioned, in bits and pieces, all through the Old Testament. One passage where the whole quartet appears:
This is what the Lord Almighty said: "Administer true justice; show mercy and compassion to one another. Do not oppress the widow or the fatherless, the foreigner or the poor."