MSOcache is a folder created by Microsoft Office. It is hidden by default so you can only see it if you change the settings of windows explorer.
This folder is created during Office installation and all the installation files are stored there. When you try to use a feature that is not installed the the new feature will be installed from this folder so that you do not need to use the original CD.
When you update Office this folder is also updated, if you uninstall the update it can be reinstalled from MSOcache folder.
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