MS Outlook uses PST (Personal Storage Table) file format to store your mails and other mailbox items. There is a limited size of PST file and if it exceeds that size limit, then the file becomes corrupted.Microsoft outlook does not provide any facility to directly split large pst file into small pst file.  

MS Outlook uses two types of PST files, one is ANSI PST and the other is Unicode PST.

1. The ANSI PST file format has a size limit of 2 GB and was used with MS Outlook 2002 and earlier versions.

2. Unicode PST file provides up to 20 GB size limit, which is used with MS Outlook 2003 and its later versions.

Heavy and Oversized PST contributes equally in making PST management challenging. As size of PST increases more than its limitation then users have to face lots of problems.Outlook users need to manage their PST file size properly time to time. Reduction of PST files size necessary for better file management. But as you reduce PST file size, there would be possibility of data loss.

Don't Worry Reduce PST file Size without any loss!!

Splitting a pst file can be done wtih Archive feature in Outlook. But split through archives prcoess can take yearly (or any other amount such as quarterly, or every 2 years.

I've had the same problem with oversized PST some time ago, so I found best Split PST Tool – Split Large PST Files into Small Ones divides PST file into manageable and smaller PST files as per the specific criterion like size, date, folder and email id, and protects them from any corruption issues because of file size limits.Performs risk-free operation of PST file splitting without making any change on the content or structure of original PST file and the original formatting or RTF and HTML messages.
 
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The best option here would be to converts the .ost backups to .pst,

I once tried KdeTools for OST to PST software and it was quite helpful. The ost2pst software helped you in restoring corrupt all OST data and converts OST to PST file format in just a few minutes along with all items. 

http://ost2pst-converter-free.software.informer.com/

Hope it'll helpful for this suggestion ...!

If you have multiple PST files and you want to merge them in one PST file, try PST Merge tool to easily complete this task with simple mouse click. It supports both ANSI and UNICODE PST file and merge mulple PST files in one PST file without any data loss.

https://www.g2crowd.com/products/merge-pst-tool/details

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