Is Opera set as your default browser? Depending on where or how the adware/malware was installed, it may only infect the default browser, whichever that is. If the adware/malware only affects Opera regardless of whether it's the default browser, the infection may be localized to Opera as an unsought extension or search engine within Opera. It could also arise from an infected session or tab if you have Opera set to resume from earlier sessions and tabs.
Try browsing from within Opera's Private window to see if the infection appears. Check your extensions within Opera and remove anything you don't recognize. Also check for unknown/strange search engines and try deleting them. Examine any Opera shortcut's Properties panel for an appended URL that might be directing the browser to a malicious site. Clear Opera's 'browsing data' via: Menu - History - Clear browsing data. Try deleting Cookies: Menu - Settings - Privacy & Security - Cookies - All cookies and site data... - Delete all - Done.
