This article explains how to download a web page in Opera to save it offline. Instructions include saving an entire page, saving a single file of a web page, or saving the HTML file.
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The quickest way to do this is to press the keyboard shortcut Ctrl+S (Shift+Command+S on macOS) to open the Save As dialog. Choose the type of web page you want to download and press Save to download it.
The other way is through the Opera menu:
Select the red O in the upper left corner of the browser.