How do I force Opera to switch over to the tab I opened when I click on 'Open in new tab'? (I am opening a new tab because I want to see the content - opening it without switching to it seems useless to me)
The "old" Opera (Presto, up to version 12.x) switches to new tabs by default and gives you the option (context menu for links) to both "open in new tab" and "open in background tab", so I'm guessing you mean the newer version of Opera (Blink, version 15+).
While playing around with and customising these sorts of settings was a breeze in the old version, the new one requires you to install extensions. You could try the Classic Tabs extension to bring back some of Opera 12's tab options, or, if that's not to your liking, use the Download Chrome Extension which lets you install Chrome extensions in Opera, and then try Tab Activate or Tabs to the front.