First, you need to remember what an apostrophe is in literary terms. An apostrophe is a passage in a text in which a character who is actually "present" addresses a person or an entity that is not physically present in the same place where the speaker is.
In the Nun's Priest's Tale, the rooster, Chanticleer, is telling his story about why it is important to take dreams seriously. He describes the traveler who dreamed of his friend being murdered. The traveler ignored the dream at first, but then found out that his friend had been murdered in exactly the way the dream said.
Having told this story, Chanticleer says
“O Blessed God, Who art so true and deep!
Lo, how Thou dost turn murder out alway!
This is an example of apostrophe. God is not physically present when Chanticleer is speaking. So when Chanticleer addresses him, it is an example of apostrophe.