It could be argued that Jesus appeared a couple of times in the Old Testament:

#1. to Abraham - God came to Abraham as a man to deliver the promise of Isaac as well discuss the destruction of Sodom

#2. to Joshua - He appears as the General of the Lord's army, Joshua falls to his in worship.

#3. To Nebunezzar - He appeared as the fourth person in the fiery furnace with Shadrack, Meshack & Abednigo

All of which were instances where God appeared to men in human form. Whether or not this was Jesus could be debated however, the bible does say in John that Jesus was with God in the beginning.