Home/PostalMag/Post Office Photos/St. Onge, South Dakota Post Office Post Office Photos St. Onge, South Dakota Post Office PostalMag · Thursday, December 12, 2019 The St. Onge, South Dakota Post Office serves ZIP Code 57779. Photo by Thomas Rosner, some rights reserved. Photo shared under the Creative Commons License. St. Onge Post Office 176 Purl St St Onge, SD 57779