You could set your monitor as a default device and just drag the Opera Window to the left. If the screen is disabled, it should open up on the main one. Also, the command for dragging the window is very simple. Select the wanted application, hold Win key + desired arrow, press a couple of times and it should appear. My combination is "Win + right arrow," and after a couple of arrow presses, it seems on my main monitor. If that doesn't help, another combination is "win + shift + arrow key". It should drag the windows across the screens. Hopefully, this helps you!