There are clear hints in the Old Testament about loving one's enemies. There are such passages in the book of Ecclesiastes and the book of Exodus, etc.

However, it must be said that the Old Testament does not address this point in a way that is as unambiguous as Jesus' statement in the New Testament.

However, the book of Jonah seems to be the best example of loving one's enemies. According to the Jewish history, the pagan nations of the Babylonians, Assyrians, and Philistines attacked Israel many times, and the first two groups took the Israelites into exile in large numbers. Therefore, the people of Israel had an unsatiable hatred and enmity towards the above three nations, especially the Babylonians and the Assyrians. God commands Jonah to go to one of those enemy nations, Assyria. Upon hearing this from God, he flees to Tarshish. When he is caught on the way and literally carried in there, he is looking forward to the destruction of the people of that land. But God rebukes him and declares that they too are his beloved.

Isn't this what Jesus is referring to when he says, when being asked for a sign from heaven? No other sign will be given than the sign of Jonah. "For as Jonah was a sign to the Ninevites, so will the Son of Man be to this generation." (Luke 11:30)

Just as Jonah was 'three days in the belly of the fish' and then came out and preached to the people of Nineveh, so the Son of Man, when he would come out of that 'great fish' on the third day, will become a sign not only to the Jews but also to all the "Ninevites"; he will become a sign of mercy!