Word Online can display math formula created using the builtin equation editor, but OneNote online cannot. After this many years MS bought OneNote, it is surprising that OneNote is still not fully integrated into the Office family.
Thank you for your suggestion, note that OneNote Online can now render equations. Editing will continue to be tracked by https://onenote.uservoice.com/forums/327183/suggestions/17797918.
Ralph Buchal commented
Not being able to view equations in OneNote online is a very significant limitation for content that has a lot of math. All content should be visible on every platform, even if it is not editable.