United States Government--post Office, For Zip Code Information Onawa, IA 51040 Phone : (712) 423-2811 Business Category Government Offices-Federal Post Offices Government Offices & Public Schools Post Office - Onawa 1017 Iowa Ave Onawa, IA 51040 Phone : (712) 423-2811 Business Category Post Offices Toll-Free           (800) ASK-USPS United States Government--u S Post Office 1017 Iowa Ave Onawa, IA 51040 Phone : (712) 423-2811 United States Government--u S Post Office Business Category Government Offices-Federal Post Offices Government Offices & Public Schools