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:
Hackers emptied ethereum wallets by breaking internet infrastructure - The Verge
Get tab suspender ext.
Also some extensions harvest RAM and like never releasing, check Opera's task manager to see what extensions are high on RAM/cpu consumption then just quickly disable and enable the fat ext. U can do that instead of restarting the browser.
GPU task also is a fat hoarder sooo sloooow to release any: kill it it will be back up in seconds but much more lightweight.
Perhaps Chromium based browsers should have tighter weight watching!? Veganize em!