In Opera, clearing the cache is done via the Clear browsing data section that's part of Settings. Once open, check Cached images and files and then click or tap Clear browsing data.
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The fastest way to bring up the Clear browsing data window is via the Ctrl+Shift+Del keyboard shortcut.
Without a keyboard, click or tap the main menu button (the Opera logo from the upper-left hand side of the browser), then Settings, Privacy & security, and finally the Clear browsing data button. Check the Cached images and files option and then press Clear browsing data.
See How to Clear the Cache in Opera [help.opera.com] for detailed instructions.
TIP: Be sure to choose the beginning of time option at the top so you're sure to erase everything!
You can clear the cache from the mobile Opera browser, too. Tap the Opera icon from the bottom menu and then navigate to Settings > Clear to choose what to delete: saved passwords, browsing history, cookies and data, or all of it.