When you save files to OneDrive in MS Office 2013 without the OneDrive desktop app, Office will open the server destination for you rather than the local synced folder. That means your document would be uploaded by Office to your OneDrive directly, and if you want to see it again on your computer, you must download again! The OneDrive desktop app fixed this problem. However, when you go to OneDrive’s website seeking for a solution for Windows 8.1, you will be notified that

"OneDrive (currently SkyDrive) is already built in. Just open the app and sync your files across all of your computers.

See how to get started"

This is confusing, right? At least to me. Indeed, the built-in OneDrive feature in Windows 8.1 (or with Update 1) ABSOLUTELY DOES NOT contain anything about integration with Office 2013. Microsoft offers an explanation and solution only here (http://windows.microsoft.com/en-us/onedrive/office-2013):

In Windows 8.1, you can install theOneDrive desktop app to install a setting that lets you use Office to work on OneDrivedocuments with other people at the same time. Because OneDriveis built in to Windows 8.1, no other desktop app features will be installed. Download the free desktop app

Anyway, if you want to use Office 2013 in Windows 8.1 with OneDrive, you’d better install the OneDrive desktop app as well.