If you are looking for the ‘common denominator’ of Office 365 available fonts / typefaces below a screenshot of comparison between Outlook Online and Word Online is provided.
You are asking “Why comparison?” Well, it turns out, that different online applications have a different subset of typefaces. So if you want to create items such as emails and word documents purely online, i.e. through the web browser, AND you need a consistent look and feel (which in my humble opinion is a MUST within the corporate identity concept for every E1-E4 subscriber), you’d have to choose a font that is supported in each app.
So there we go:
PS: If you need to be consistent in Excel and PowerPoint as well, the subset is getting even smaller, but I don’t have the time and resources to make such comparison.
PPS: If you need that your design be perceptible across all platforms – Winows, Mac OS X, Linux, you will be left with even smaller subset. Either search online for common web fonts or have a look here.
PPPS: The ‘unknown’ fonts were in Asian, sorry, cannot help there.