There are slight feature differences between the Office programs installed on your PC and their Online counterparts. This guide from Microsoft’s Knowledge Base outlines those differences.
Any phone number files (.xls, .ppt, .doc) cannot be edited with Office Online.
Box for Office Online now supports real-time co-authoring. Multiple people can edit a single document simultaneously, and all changes will be automatically saved back to Box. When multiple people are editing a document, their names will appear in the upper right corner. Your collaborator’s text cursor will appear in a unique color, and their name will be displayed next to it.