Hi @oldfart1, you can use Ctrl+Shift+Click, or Shift+Middle-Click to open a link in a new tab in the foreground, and Ctrl+Click, or Middle-Click to open link in a new tab in the background.
You can also use mouse gestures for that: Move down, to open a link in a new tab in the foreground, or move down, then up, to open a link in a new tab in the background - to enable it, go to Settings > Browser > Shortcuts, and tick the checkbox on the left of "Enable mouse gestures".
Alternatively, you can drag and drop a link on a free space on the tab bar, to open it in a new tab.
Finally, if the only acceptable option for you, is to use left click for that purpose, you can install the "Long Press New Tab" extension. To be able to install it in the Opera, you need first install one of the following extensions: the "Extension source viewer" , or "Download Chrome Extension".
After you install it, you can select several options, e.g. the long-press duration, or force to open links in a new tab in the foreground 😉