Sometimes I need to open two pages at once to make an comparison. I normally open two tabs and just switch back and forth. This time I decided to try and open one of the tabs in a new window. I right clicked on the tab to see if there where any options. Then the web page, but nothing. So I did this.
I clicked on the top tab of the web page that I wanted to open in a new window, and held down, then I dragged it over to my second monitor and let go. You'll get a little thumbnail view of the page in question. And it worked. If you don't have a second monitor just drag the thumbnail of the second web page down a bit and it let go. Just a note: sometimes you don't see the full thumbnail view but it should still work, also this closes out the tab (web page) in the original window.
Now you got two copies of Opera open. So just snap them left and right to your monitor. To snap it to your monitor just drag it quickly to either the left or right hand side and let go.
If you want to open a bookmark or a link on a web page to a new window then you do have the option just to right click and then click on Open link in new window.