@rmarinrojas If Opera is not set to autostart on your system whenever you power the system up, and if you've set the default browser to another non-Opera browser, and if a system restart immediately brings up Opera at the gambling website, you probably have adware present on the system which is triggering from one of the Windows startup folders - either at the system level or at the user account level.
As a simple first try at a fix, after restarting your computer, press Windows' Start button, and go to Settings. Then in the "Find a setting" box in the middle of the page that appears,, type: Startup. From the popup menu that then appears, click on Startup Apps. You should see a listing of apps or programs that the system and the user account activate at system startup. Scan the list for anything that you don't recognize as belonging there - especially something that may refer to Opera or the gambling website. If there is something alien there, use Windows' "Add or Remove Programs" function, locate the alien name again on that list, and uninstall it.
Then use Explorer to go to the folder at C:\users\<your username>\AppData\Roaming\Microsoft\Windows\StartMenu\Programs\Startup and look at the filenames listed there... again, look for something that you don't recognize as belonging there. If there's something alien there, delete it.
Otherwise, you may want to want to download and try some reputable adware removal tools like AdwCleaner. For persistent adware problems, you may need to download and employ multiple cleanup programs as discussed at the Bleeping Computer website: https://www.bleepingcomputer.com/virus-removal/how-to-remove-adware-on-a-pc