The costumes on the show are designed by Eduardo Castro. They have been put on display at several exhibitions, including at the Fashion Institute of Design and Merchandising (FIDM), that you can see photos from below. For the episode "Hat Trick" the costumes got nominated for an Emmy.
Most of the characters' costumes resembles the costumes from the Disney versions. Such as Pinocchio's costume, Cinderella's blue gown and glass slippers in "The Price of Gold", Belle's dresses in "Skin Deep" and Snow White's costume in the opening scenes of "Heart of Darkness".
On the DVD for season one, you can learn more about the costumes, on the bonus feature "Building Character".
Once Upon a Time costumes at the FIDM's Art of Television Costume Design ExhibitEdit
Photos thanks to The Huffington Post