@Cha0sCharly after reading this thread & some Reddit threads, I tried shutting off anti-virus, making sure all Windows Updates were current, etc. Did not help. I isolated the problem to corrupt sessions files. Close Opera. Find your sessions file folder.
Mine were at C:\Users[user]\AppData\Roaming\Opera Software
Delete all the session files. Reopen Opera. It will create new session files and should open normally. Be warned, you will likely lose all browsing history, tabs, bookmarks, and workspaces. If you have Opera Sync, you can restore bookmarks, etc. But you will not be able to restore workspaces.
Also, from now on, I will be backing up sessions files regularly.

When using Opera One, I usually have a lot of pinned tabs & open tabs. I can't easily tell at a glance which tab is the one that I currently have open, other than the icon is slightly raised & white. Is there some sort of setting that I can use to better highlight the open tab? Maybe put a red ring around it or something?