You cannot do this through the browser. So to achieve what you’re wanting to do, you’ll have to use a different application like MyCrypto. Download the MyCrypto app and verify it’s authenticity to prevent any phishing and then you’ll put in your mnemonic from opera and it’ll give you a list of addresses to choose from along with the ETH balance of each address.
Select your desired address and then you’ll see a wallet info tab. Click view private key and there you go, just copy it and import it into whatever other wallet, app, etc you need
It’s generally believed that downloading the MyCrypto app instead of using the online tool because of events such as this: