Our add-ins work in the desktop version of Outlook, but will not appear in the web interface of Outlook.
That said, there’s nothing wrong with having an Office 365 account as your email server and using the desktop version of Outlook. The add-ins work great under that scenario.
Finally, we do have versions of some of our add-ins that work directly in Office 365. These are mostly subscription versions of the add-ins and can be found at on our Office 365 subscriptions product page.