When you open the Crypto Wallet from Settings in Opera for the first time it is possible to import a wallet instead of creating a new one. We import the wallet using a seed phrase usually referred to as a Mnemonic. It is the same thing that is used by Metamask.
If you've already set up a wallet you can back its mnemonic up (preferably on a piece of paper), it is available in the wallet settings menu. Once the key is safely backed up you can press the "Sign out from wallet" option in the wallet settings and import your metamask wallet using its mnemonic.
Opera for Android