People across Japan cast ballots on Sunday to choose local leaders and assembly members as well as two Lower House lawmakers.
National parties campaigned fiercely, framing the poll as a bellwether for the Upper House election in the summer.
In Okinawa, a candidate who opposes the government's plan to relocate a US military base is projected to win the by-election.
Freelance journalist Tomohiro Yara is backed by five opposition parties and a local party in Okinawa.
He is projected to defeat Aiko Shimajiri of the main ruling Liberal Democratic Party. Shimajiri is supported by the LDP's coalition partner, Komeito.
Vote counting has just started, but NHK's decision desk is already projecting some winners based on exit polls and other information.
The Okinawa electoral district includes the planned relocation site for the US Marine Corps Futenma Air Station. The Japanese and US governments want to move the base from a densely populated area to a less crowded coast district. They say that's the only way to reduce safety hazards for nearby residents while maintaining the deterrence of the Japan-US alliance.
But Okinawa Governor Denny Tamaki opposes the plan and wants the base to be moved out of the prefecture altogether.
The by-election was held to fill the Lower House seat vacated by Tamaki when he ran in last year's gubernatorial election.
Four candidates are running in the by-election in Osaka.
NHK's exit poll shows that Fumitake Fujita of Nippon Ishin is in the lead. The party is advocating what's known as the "Osaka metropolis plan," which would abolish Osaka City and reorganize it into special wards just like Tokyo. The party says their plan will cut costs by streamlining overlapping city and prefectural services.
People across Japan also voted in 721 elections to choose mayors and local assembly members on Sunday.