All the Jews knew the OT before the NT was written. Jesus often quoted from the OT, and His Jewish listeners understood His references. Even many non-Jews new the OT. Example: the wise men who followed the star to Jesus; they weren't Jews but they knew the prophecies of the OT.
Jesus commissioned His apostles to spread the good news of the Gospel, and to teach His words. Their focus was what we call the NT. That doesn't mean ignore the OT, or make it less important. But not as many people knew about Jesus and Christian life at that time, so that was the apostles' goal, to spread that news. It wasn't their responsibility to spread the OT.