Having the same issue with Opera Beta 25.0.1614.35 and Yosemite developer preview 5. It's crashing even when I do nothing and Opera sits in the background doing nothing. Something around 4-5 times per hour. Tried disabling/removing all extensions I have. No luck. Haven't tried with a Guest account yet.
Here is a crashlog of the latest crash I had a couple of minutes ago: http://pastebin.com/rHKCVWU8