With the preview release of "the new Outlook" on OWA, Office Add-ins are now buried in the drop down context menu in an email. Additionally, it appears they do not follow the same behavior of Outlook on the desktop, where Office Add-ins will show in the larger ribbon if used more frequently.

According to this article, Office Add-ins can be pinned if they are used frequently:

If you were using add-ins in the classic Outlook on the web experience, they will show up pinned next to mail actions or in the drop-down menu once you open an email in the new experience. You can also install more add-ins from the Office Store—simply open an email, click the drop-down menu, and select Get Add-ins. Once installed, add-ins will show up in the drop-down menu. You can also choose to pin frequently used add-ins.
While I can instruct users to pin this for themselves, it does not appear that Office Add-ins can be pinned by an administrator.

For this feature, it would be great for administrators to be able to customize which add-ins are immediately available to the user by pinning them. Office Add-ins being buried in the menu of OWA is not helpful in this regard.