the old testament aka torah was the old covenant that God made with his chosen people of israel.
"Behold, the days come, says the Lord, that I will make a new covenant with the house of Israel, and with the house of Judah: not according to the covenant that I made with their fathers in the day that I took them by the hand to bring them out of the land of Egypt; which my covenant they broke, although I was a husband unto them, says the Lord: but this shall be the covenant that I will make with the house of Israel; after those days, says the Lord, I will put my law in their inward parts, and write it in their hearts; and will be their God and they shall be my people. And they shall teach no more every man his neighbor, saying, Know the Lord: for they shall all know me, from the least of them unto the greatest of them, says the Lord: for I will forgive their iniquity, and I will remember their sin no more" (Jer. 31:31-34)
prophets of the old testament knew God was going to establish a new covenant
"For if that first covenant had been faultless, then should no place have been sought for the second" (Heb. 8:7)
people in the OT offered sacrifices to pay debt of their sins, Jesus came and established the new covenant and became the sacrifice for all of man's sins so just by believing in Him u will receive salvation.