I found a work-around that would probably help with other Google apps that don't work right with Opera. Apparently, Google deliberately makes it difficult for some apps to work with Opera since they want to force Opera (i.e. Google Chrome variant) users to switch to Chrome. (How nice of them!). I found a user-agent switcher add-on for Opera that is similar to the old Opera option that allowed Opera to be seen as another browser.
Here is the link: https://addons.opera.com/en/extensions/details/user-agent-switcher/?display=en
This add-on installs a toolbar button (similar to what ad-block does) that allows you to set Opera to mimic a wide variety of browsers. I added it to Opera Stable, opened Google Drive, and got the pop-up about using an unsupported browser, but was able to click on the X to close the pop-up, after which Google Drive looked fine!!