New census figures indicate the population on Oʻahu fell by tens of thousands over the previous eight years.
Census data from 2010 to 2018 shows Oʻahu's overall population decreased by 62,000.
Net migration loss on Oʻahu from 2016 to 2017 was more than 14,000, while about 13,000 more residents departed than arrived between 2017 and 2018.
A county population previously headed for 1 million dropped from nearly 993,000 in 2016 to 980,000 by 2018, figures showed.
No other county experienced a population drop close to Honolulu County, which includes all of Oʻahu, with Maui County the only other showing a net migration loss.
Oʻahu's high cost of living and employment in mainland states were likely factors, officials said.
"The mainland in recent years is booming with more job opportunities," said Eugene Tian, Hawaii's chief economist.
Military deployment also contributed. Oʻahu's military population decreased from nearly 50,000 in 2014 to about 43,000 in 2018, according to the state Department of Business, Economic Development & Tourism.
Hawaiʻi's state population also fell from nearly 1.43 million in 2016 to 1.42 million in 2018.
Honolulu County now makes up 69% of the state's population, down from 72.3% in 2000. The Big Island makes up 14.1% of the state population, while Maui County has 11.8% and Kauai County has about 5%.
Tourist numbers have gone in the opposite direction. Visitor arrivals to Hawaiʻi reached an all-time high in 2018 for the seventh year in a row.