There are a host of verses to which one could look throughout all of the Old Testament, with the first being in Genesis 3:8-21, God could've brought complete destruction on Adam and Eve, but instead showed mercy by allowing them to live and gave them a promise of a redeemer who would eventually come to destroy the serpent.
Genesis 4:6-7 shows God having compassion on Cain before he killed his brother:
Then the LORD said to Cain, "Why are you angry? And why has your countenance fallen? If you do well, will not your countenance be lifted up? And if you do not do well, sin is crouching at the door; and its desire is for you, but you must master it."
Jump ahead to the conquest of Canaan, and see the mercy and love shown to Rahab and her entire family (she is so important for having been saved in Jericho that she is in the human line of Christ):
"Now therefore, please swear to me by the LORD, since I have dealt kindly with you, that you also will deal kindly with my father's household, and give me a pledge of truth, and spare my father and my mother and my brothers and my sisters, with all who belong to them, and deliver our lives from death.? So the men said to her, "Our life for yours if you do not tell this business of ours; and it shall come about when the LORD gives us the land that we will deal kindly and faithfully with you."
She's even celebrated in the New Testament, "By faith Rahab the harlot did not perish along with those who were disobedient, after she had welcomed the spies in peace."
In Daniel 3:28-30, we see God's deliverance of Shadrach, Meshach, and Abednego as having a profound impact on the king:
Nebuchadnezzar responded and said, "Blessed be the God of Shadrach, Meshach and Abed-nego, who has sent His angel and delivered His servants who put their trust in Him, violating the king’s command, and yielded up their bodies so as not to serve or worship any god except their own God. Therefore I make a decree that any people, nation or tongue that speaks anything offensive against the God of Shadrach, Meshach and Abed-nego shall be torn limb from limb and their houses reduced to a rubbish heap, inasmuch as there is no other god who is able to deliver in this way." Then the king caused Shadrach, Meshach and Abed-nego to prosper in the province of Babylon.
In Esther we see God strategically putting one of His people into a position of extreme influence to the King whereby He saves His people from the maniacal plans of Haman who wanted to exterminate the Jews.
In II Kings 5, we see God having mercy on the captain of the Assyrian army, Naaman via the instrument of a Hebrew maid girl in his house.
In short, the Old Testament is as replete with God's mercy, love, and compassion as the New Testament.