In Opera, goto the URL opera://about and take note of the "profile", "cache", and "install" paths. Then, close Opera.
For good measure, delete the "cache" folder in the cache path even if you cleared cache in Opera while it was running.
In the profile folder, check the size of the "File System" folder to see if it's what's taking up so much space. If so, start Opera, goto the URL opera://settings/content/all and remove any entries that are using file system storage. These will most likely be Mega Download sites. Then, close Opera and delete the "File System" folder for good measure.
In the install folder delete all the old version folders. They might have piled up.
Delete everything in "C:\Windows\temp" and "C:\Users\yourusername\AppData\Local\temp". Old installer stuff can be in there.
Then, if you want, see https://peter.sh/experiments/chromium-command-line-switches/#disk-cache-size. You can launch Opera with a command-line switch (via a command prompt or the run dialog or by modifying Opera's shortcut) to limit the cache directory size.
As for automating the process of clearing things, maybe https://chrome.google.com/webstore/detail/auto-clear-browsing-data/bfgoiiandhlddbcenmpijfpacgkdeenp will help. Never used it though.