“O Lord.” This is the Hebrew Adonai, and it is stronger than the word for Lord in Judges 6:13. Gideon’s recognition of who he is dealing with is deepening and that is reflected in the way he addresses the messenger of Yahweh.
“how can I save Israel?” Moses started the same way, with much doubt and God had to say that He would be with Moses (cp. Exod. 3:10-12).
“least.” This could refer to the least significant or even to the youngest member of the family.