Opera does support custom site panels in the sidebar though. Just goto the URL opera://flags/#sidebar-site-panel, enable the flag, restart opera, click the 3 horizontal dots at the bottom of the sidebar, scroll down to "Custom Site Panels", and add https://reddit.com/ as a panel. You could do it with the MS Office URL too.

Now, you can kind of (very loosely) simulate an app in Opera like so:

Goto the URL opera://about and take note of the "install" path.

In Windows Explorer (File Explorer), goto that path, right-click on launcher.exe and choose to create a shortcut on your desktop. Then, right-click the shortcut and rename it to "Reddit App".

Then, right-click on the shortcut again, goto "properties", switch to the "shortcut" tab, and modify the target field command to add a space and:

"--user-data-dir=%AppData%\Opera Software\Reddit App"

after the quoted launcher.exe command.

Then, double-left-click on the desktop shortcut to load it.

Then, you're going to want to customize that Reddit App Opera profile.

Goto the URL opera://settings/onStartup, switch to "Open a specific page or set of pages", and add https://reddit.com/ so that Reddit opens up when you start.

Then, click the Easy-Setup button on the right off the address bar. You're going to want to disable the sidebar at least. You can enable adblocking if you want.

Then, goto the URL opera://settings and disable all the things you don't need enabled.

On opera://startpage, you can delete every speed dial. Then, you can goto the URL opera://bookmarks and empty the bookmarks trash.

You can goto the URL opera://settings/startPage, hide the search box, and disable everything else if you want. You can also install https://chrome.google.com/webstore/detail/new-tab-start-page/mjfahldkpjhcnfbbmdhpjolcjpcfhcpj to set the new tab page to https://reddit.com/ if you want.

To delete the Reddit App's data, goto the URL opera://about in it and take note of the "profile" and "cache" paths. Then, close the Reddit App and delete those folders. To delete the app itself, just delete the desktop shortcut.

Besides all the downsides of manually setting that up, you're not going to get a minimal user interface. But, you could shrink the windows's width to as small as it will go and then adjust the windows's height to what you want. Silly, but there you go.