I think you want to reduce the size of OST file in Outlook. So use the eSoftTools OST to PST converter software and convert Outlook OST all mailbox items to PST. After conversion select the PST splitting option where you can esiy split a large PST file into multiple small-size PST files without any errors. Technical and nontechnical users can easily operate this tool without any misunderstanding because this tool designed with a friendly GUI.