Omaha, Nebraska is sometimes called the ‘Big O.” Basketball superstar Oscar Robertson (an NBA player from 1960 to 1974) popularized “The Big O” nickname for himself. New York City’s “Big Apple” advertising campaign began in the early 1970s.
“Big O” (for Omaha) was cited in citizens’ band (CB) radio slang from at least December 1975. “‘Big O’—Omaha”—not in CB slang—was cited in the Sunday World-Herald (Omaha, NE) in September 1977.
Omaha was nicknamed the “Magic City” in the 1880s.
Wikipedia: Omaha, Nebraska
Omaha (/ˈoʊməhɑː/ oh-mə-hah) is the largest city in the state of Nebraska and the county seat of Douglas County. Omaha is located in the Midwestern United States on the Missouri River, about 10 miles (15 km) north of the mouth of the Platte River. Omaha is the anchor of the Omaha-Council Bluffs metropolitan area, which includes Council Bluffs, Iowa, across the Missouri River from Omaha. According to the 2010 census, Omaha’s population was 408,958, making it the nation’s 43rd-largest city. According to the 2014 Population Estimates, Omaha’s population was 446,599. Including its suburbs, Omaha formed the 60th-largest metropolitan area in the United States in 2013, with an estimated population of 895,151 residing in eight counties. The Omaha-Council Bluffs-Fremont, Nebraska-IA Combined Statistical Area is 931,667, according to the U.S. Census Bureau’s 2013 estimate. There are nearly 1.3 million residents within the Greater Omaha area, comprising a 50-mile (80 km) radius of Downtown Omaha, the city’s center.
17 December 1975, New York (NY) Times, “Citizens’ Band Radio: A Growing Fad,” pg. 90, col. 1:
Each time he embarks on a highway trip—say, from his hometown of Lincoln, Neb., to Omaha—Earle Munns turns the citizens’ band radio in his car to Channel 19, identifies himself as “Paddlewheeler” and asks for a reading “eastbound on Rocky between here and Big O.”
Jason’s Authentic Dictionary of CB Slang
By Wayne Floyd
Fort Worth, TX: Jason Press Company
Big O Omaha, Nebraska
18 September 1977, Sunday World-Herald (Omaha, NE), “Immigrant Author Drifts Back to Dana” by Tom Allan, pg. 17-B, col. 1:
“Big O”—Omaha—is the central city in the story which he describes as a “human action tale wiih a tragedy at the end.”
And an hour later we were towing old Econoline up highway 29 to the “Big O,” discussing the possibility of just buying a tow truck and towing ourselves around the country.
How to Talk American:
A Guide to Our Native Tongues
By Jim “the Mad Monk” Crotty
Boston, MA: Houghton Mifflin Company
the Big O: Omaha, my revered place of birth.
By Janet Kuypers
Chicago, IL: Scars Publications
Doug told us tonight while we were at dinner that people from Omaha refer to their town as “The Big O.” I don’t know if he was joking with me or not, but when I think of “The Big O,” I think of something entirely different.
OCLC WorldCat record
Big “O” playas. Vol. 1
Publisher: Omaha, NE : Exclusive Sound Records, 
Edition/Format: Music CD : CD audio : Popular music : English
back to Omaha tomorrow, been there more times in the last two months than the last five years, I do love the big O
4:47 PM - 27 Oct 2008
Trocadero welcomes everyone coming to the Big O! for BIG OMAHA!!
3:16 PM - 12 May 2010
OCLC WorldCat record
Uniquely Omaha : 101 things to know about the Big O
Author: Michael Kelly; Omaha World-Herald Company.
Publisher: Omaha, NE : Omaha World-Herald Co., 2015 ©2015
Edition/Format: Print book : English : First edition
@griph Also, Omaha often calls itself The Big O.
11:30 AM - 1 Aug 2016
Nicknames of Other Places • Big O, Magic City (Omaha nicknames) • Wednesday, August 03, 2016 • Permalink