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.
eSoftTools ost to pst converter helps convert ost files to pst and other formats and also help to split large pst files into small ones without losing data. It is easy to use due to its user friendly interface.