OST file is an Outlook data file created by MS Outlook application for windows to store all kind on emails, contacts and other data related to your email account. This is an offline backup of your email data. Latest versions Microsoft Outlook email client has a default size limit of 50GB for the OST file. If the OST file size crosses this limit then it affects the performance of Outlook app.
To fix and large-sized OST file we can delete permanently your emails in deleted items folder or archive your OST file. But, if the size of the OST file still above the size limit, you need to split OST file into small sizes.
There is no direct method in Outlook software to compress or split the OST file into multiple small OST files. But you can use a third-party tool to perform this action. vMail OST to PST Converter is an email conversion tool for MS Outlook that directly converts OST file into PST file. This tool is capable to convert the unlimited large size of OST file into PST format and you can also use this software to split the large PST file converted from OST to split into your choice of multiple small size of PST files.
Learn More- How to split OST file?