There are many file conversions needed during the O365 Migration process which acts as a hindrance if not done carefully. One of those hindrances is OST to PST extension conversion.

Method 1: Export mailbox as a PST file Login to Outlook From The Microsoft 365 or 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.

Select Export to a file; then click Next.

Select the second optionOutlook Data File (.pst)and click Next

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

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

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 the 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.

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.

Hope these methods work out for you.