Please discontinue Excel Online
Look--there is Excel on a user's desktop that has one set of rules, then there's Server versions of Excel sheets that have a different set of rules. Now Excel Online has an entirely different (and cripplingly limited) set of rules.
So if you must continue the madness--at least call it something else, like "Doodlebumpkins" or "Excel Lite".
Calling it "Excel" severely diminishes the brand. People who have only used Excel in the "online" version will hate it forever and would be unlikely to ever purchase the "real" Excel because you don't get a second chance to make a good first impression.