Sir Orfeo is an example of Middle English romantic literature—or a "Breton lai," as it is called by scholars. Breton lai were influenced by French chivalric narratives, which usually featured story elements such as magic and love. Celtic touches were also common, particularly those of fairies or a fairy land. Due to the nature of Sir Orfeo's literary genre and the culture within which it was created, it is quite different from the original Greek myth of Orpheus and Eurydice.

Firstly, Sir Orfeo has Celtic elements such as fairy characters. The villain is now a fairy king who spirits Eurydice away to his underworld kingdom, while in the original myth, death itself is what separates the lovers.

Secondly, Sir Orfeo changes the roles of the characters. Orpheus the poet is now Sir Orfeo the king. Eurydice is now called Heurodis, and she is Orfeo's queen rather than his doomed bride.

The most significant change is the ending. In the original Greek myth, Orpheus disobeys the one rule he was given in exchange for Eurydice's full resurrection: that he not look back at her until they left the underworld. Then Orpheus himself dies. In Sir Orfeo, the two lovers escape the fairy world and are reunited in their kingdom, allowing for a happy ending more appropriate for the Breton lai genre.