You should be able to goto the URL opera://settings/content/all and see what sites are taking up that much data and then delete the data for those sites. Or, you can goto the URL opera://settings/clearBrowserData, switch to "advanced", check only "cookies and other site data", set time range to "all time" and click "Delete data" to clear data for all sites.
You can also clear "cached images and files" to clear your cache.