S4 finale had a Shrek-y vibe to its plot containing the princess locked in a tower guarded by a dragon plus a hero on a quest to get everyone's memories back - which they lost due to the machinations of a version of Rumplestiltskin - by means of a true love's kiss.
Shrek does not, and will not appear, due to it being made by a direct competitor to Disney. As others have said already though, Ogres as a species appear, and certain plot elements seem similar to certain plot elements in the Shrek franchise. Enjoy watching Once, and feel free to ask us other questions!
The Only things that remind me of Shrek in this show are the tower where Emma was locked up, guarded by Black Knight Lily in s4 finale, who turns into a dragon, and the lines in 2x03 where they talk about the differences of ogres and giants.