I've given this a quick test and sure enough the offline Excel file no longer goes corrupt when I edit it online.
Existing offline corrupt files do not automatically repair themselves, however, a simple edit to any online file result will result in the corrupt version being replaced by the undamaged online version.
For example, open a damaged file online, type a space at the end of the document (or in a blank Excel cell) and close the browser window, then repeat for any other recently edited files that became corrupt offline. OneDrive will then sync the offline versions and they should no longer be corrupt.
Thanks for the update Tony,