If you want to reduce the size of an OST file in Outlook, I know the best splitting and conversion tools that's named eSoftTools OST to PST Converter software. With the help of this tool, you can convert OST to PST and 10+ formats like Office365, Gmail, Yahoo Mail, Zimbra, MBOX, MSG, ICS, vCard, EMLX, and a lot of formats. When the OST file is converted to a PST file, users can easily split a large PST file into multiple small PST files without losing any important data. To get more information visit the official site of eSoftTools.