A family of Microsoft word processing software products for creating web, email, and print documents.
The Preferences for Mac's Dictation feature are located in
> System Preferences> Keyboard- Dictation. That may vary a bit depending on specific edition of macOS but should point you in the right direction. If not in Finder go to Help> macOS Help & search using dictation as the criteria.
However, the default is F5 [Microphone] or fn+F5 depending on Keyboard configuration. I don't find the ~/ key involved even if I choose the British or British - PC Input Source, so I don't see how **Option+** could be triggering it in the desktop software unless it's been customized.
In the web app, though, Option+` is assigned to the Dictate command but the web app shortcut for that feature cannot be modified or disabled.
I'd suggest using either Contact Support or Feedback on the in the web app Help tab.