I'm on a Mac, and installed Little Snitch (firewall for outgoing connections), and noticed that Opera v19 uses Google's public DNS servers instead of what's configured in my network settings.
There is no setting to change this.
Of course, this is happening because Opera needs money from Google, and is now helping them gather information on people. This way, Google sees every single web page you open, which is exactly why they're providing their public DNS service in the first place - to gather as much information on everyone as possible. But I'm sure Opera users would not want to have their privacy invaded stealthily like this.
Let's see how long this post stays here before it's censored.