When a user edits using Microsoft Office Online, the file will be locked, and no other application (besides another Office Online session) will be able to save as long as that file is open. If someone uses Desktop Sync to edit the same file during that time, they will not be able to upload that file until it is unlocked, and at that time, the Desktop Sync will save the file and upload it into the cloud. The Desktop Sync user will cause a conflict.