Leaning on a classical ruin, Saint Joseph looks at the Virgin Mary with the Christ Child in her lap. Jesus, in turn, is receiving a roll with the words Ecce Agnus Dei from Saint John.
This Latin inscription meaning Lamb of God refers to the Passion of Christ, who sacrifices himself, just as lambs were sacrified by the Jews, in order to save humanity from sin.
The oak tree standing behind the main group gives its name to the work and separates the figures from the background landscape and the ruins of the Caracalla Baths on the left.