Preventing Opera from switching over to new tab

Don't click "Open in new tab". In Opera feature you want is called "Open in Background Tab".

But IMHO best solution is to use middle mouse button click. In Preferences → Advanced → Shortcuts → Middle-Click options you can change how middle mouse button behaves (if it doesn't open in background already).

Ctrl+Shift+ Click on the link should open it in a background tab.

but the preference to not change focus to the new tab should be under
Preferences > Advanced > Tabs

EDIT: but Gnoupi says it isn't. :(

Just Middle-click on the link will open the link URL in a new background tab. If this is not working for you, Then go to Preferences. Go to Advanced tab. Select Shoutcuts. Click on "Middle Click Options" button. Here select "open in background tab" radio option. Hope this works for you. :)