Method 1: Export mailbox as a PST file
Login to Outlook with the Exchange account that was used to create the OST files. And to export the OST mailboxes to PST, follow the simple procedure:

Go to Files >Open & Export and click Import & Export.
Import & Export

Select Export to a file; then click Next.
Select Export to a file

Select the second option—Outlook Data File (.pst)—and click Next.
Select PST file

Select the folder which you want to export and click Next.
Select the folder to export

Browse the location where you want to save the new PST file and choose the options regarding the duplicate items.
Browse the location

Note: There is an optional choice to provide a password to the PST file, but you can easily move ahead without choosing it.

Method 2: Use ‘Archiving’ to save OST emails to PST
By enabling the Auto Archive method in Outlook, you will be able to save the items of OST file in a separate PST file. This method helps you to keep the size of your mailbox manageable. When the size of the mailbox grows and there are multiple messages that are older and you do not require them to be in the mailbox, then you can put them in the archive folder easily. Follow the simple steps to enable auto archiving in Outlook.

Launch the Outlook application, go to File, and click Options.
Go to Advanced and click Auto Archive Settings.
Auto Archive Settings

Now select the number of days in which you want to run auto archiving. Select the folder to save the archived file; also, provide the archiving options and finally click OK.
Select the folder to save the archived file

With this, mailbox items from your OST file will be saved to the PST file as per your archiving settings.

I hope both these methods will be helpful!
Mark Wilson