Oppression means bringing the body, not the heart, under submission. An orphan feels inferior when he loses his father, so do not humiliate him.
Thus, the Prophet (saws) used to rub the orphan’s head and he (saws) said:
“Whoever rubs an orphans head, and does so only for the pleasure of Allah, he will receive a good deed for each strand of hair he rubs”
And he (saws) said:
“I and the one who looks after an orphan will be like this in Paradise,” showing his middle and index fingers and separating them.
 Musnad Imam Ibn Hanbal, Hadith No. 22207
 Sahih Al-Bukhari, Book of divorce, Hadith No. 4998