If you want to reduce the size of OST file in Outlook so try the most effective conversion tool that's named eSoftTools OST to PST converter software. It has a splitting option, using this option you can easily split a large PST file into multiple small sizes PST files for example 1GB, 2GB, 3GB, 4GB, 5GB, and 10GB. This tool is designed with a simple (GUI) Graphical User Interface in this reason Any user can easily operate this tool without facing any problems.