I noticed the phrase 'O Lord' several times in Daniel's prayer in Chapter 9. For example:
O Lord, hear; O Lord, forgive. O Lord, pay attention and act. Delay not, for your own sake, O my God, because your city and your people are called by your name. (Daniel 9:19)
I am wondering is there something peculiar in the Hebrew that triggers translators to chose 'O Lord' rather than Lord. The impression I get is a deeper feeling in the prayer but am not sure if there is any formal reason for assuming anything like this or not.