For splitting Outlook PST file into multiple PST files users can use a recommended solution MailsDaddy PST Split Tool. This is the best tool for dividing the large PST file into multiple small Outlook PST files. With this utility, users can also segment the heavy PST file by Size, Folder, Emails, and Date. The software supports the PST file of all Outlook versions. This software works on Unicode and ANSI PST files. You can use the free demo version of this software and always find the best result as your wish.

To examine it before, visit here:- PST Split Tool (https://www.mailsdaddy.com/pst-split-tool/)

If your OST file is not opening then don’t worry, you can easy repair corrupt OST file and convert OST file into PST format by the manual tricks and OST to PST Converter tool.  As a Data Recovery expert I suggest all the users to read the article: How to remove errors in OST file and convert OST file into PST format? from where they will know the complete OST to PST conversion process guide which will help them to convert OST to PST in simple way.
 

If the size of PST file exceeds its limit then the PST file creates problem. Then there is only way to reduce this problem i.e. - to split the large PST file into small parts. Here I shared you the link: Manual tricks of PST split from where you will learn the 3 simple manual methods to break PST file into many PST files. This manual tricks gives the outstanding result and doesn’t damage the original data from PST file while splitting it to small PST parts.
 

If you're using Outlook Email client application where all your email saved in PST or Personal Storage Table file format then these two thechnique Split and Merge will definitely help you to come back from two major issues:

1. When have large size Outlook PST file.
2. When have large number of Outlook PST file in Outlook account to access.  

Split Technique: This technique will help you to break large size size Outlook PST file in some small PST files so that user can use data of large size file easily. 
Merge Technique: It will allow user combine and merge large number of Outlook email files in one single file.

You can take more information about both two technique from the link: Split and Merge Outlook PST file