OST (Offline Storage Table) and PST (Personal Storage Table) are two file formats used by Microsoft Outlook to store emails, contacts, calendars, and other data. An OST file is created when Outlook is configured to work with an Exchange Server, while a PST file is created when Outlook is configured to work with a POP3 or IMAP email account.

If you need to convert an OST file to a PST file, you can follow these steps:

1. Open Outlook and create a new PST file: Click on File > Account Settings > Account Settings. In the Account Settings dialog box, select the Data Files tab and click on Add. Choose "Outlook Data File (.pst)" as the file type and select a location to save the PST file.
2. Export the contents of the OST file: Click on File > Open & Export > Import/Export. In the Import and Export Wizard, select "Export to a file" and click on Next. Choose "Outlook Data File (.pst)" as the file type and select the OST file you want to export. Choose the items you want to export (e.g., Inbox, Calendar, Contacts, etc.) and click on Next. Choose the destination folder for the exported data and make sure the "Include subfolders" checkbox is selected. Click on Finish to start the export process.
3. Import the exported data into the PST file: In Outlook, click on File > Open & Export > Import/Export. In the Import and Export Wizard, select "Import from another program or file" and click on Next. Choose "Outlook Data File (.pst)" as the file type and select the PST file you created in step 1. Choose the destination folder for the imported data and make sure the "Include subfolders" checkbox is selected. Click on Finish to start the import process.

Once the import process is complete, you should be able to access the contents of the OST file in the PST file. Note that this process may take some time depending on the size of the OST file and the speed of your computer. For this conversion, you can also use the OST to PST Migration Tool to do this conversion.