There is a listing of fonts available in Microsoft Office 365 – that is Word, PowerPoint and Excel. There are the same fonts that are available in desktop applications.
This is where you find a listing of the fonts.
Cloud fonts are available to Microsoft Office 365 subscribers on all platforms and devices. Documents that use cloud fonts will render correctly in Office 2019.
You can embed cloud fonts for use with older versions of Office.
There is a useful Guide to Cloud Fonts in Microsoft Office 365 that gives you an illustration of every font plus one or two brief notes.