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Hi BK
Before I saw your question I decided to uninstall Opera and then do a clean re-install.
It seems good for now, no Yahoo hi-jacking 
I'll come back if the problem reoccurs, hopefully with more detail of any app updates that might have happened immediately before.
OK - I'm located in UK, I've just updated to 34.0.2036.25
It not only still jumps to Yahoo, but it also changed my start screen to Yahoo!
I checked my recently added apps, and the only app I've added recently was Spybot SAD, which I added after this started to see if there was any spyware causing this problem.
Previous apps were added too long ago.
Since yesterday (7/12/15) my install of Opear is also being hijacked by Yahoo. I can get to Google, but the results are being provided by Yahoo.
Anybody come up with an answer yet?