Converting OST to PST format without any cost can be achieved through various manual methods. Here are some free methods you can try:

Using Outlook's Import/Export Feature:
    Open Microsoft Outlook and create a new PST file.
    Go to "File" > "Open & Export" > "Import/Export".
    Select "Export to a file" and click "Next".
    Choose "Outlook Data File (.pst)" and click "Next".
    Select the OST file you want to convert.
    Choose the location and name for the new PST file and click "Finish".

Using Outlook Archive Feature:
    Open Microsoft Outlook and create a new archive PST file.
    Go to "File" > "Options" > "Advanced".
    Click on "AutoArchive Settings" and choose the desired options.
    Run AutoArchive to move items from the OST file to the archive PST file.

Using Outlook's Drag and Drop Feature:
    Open Microsoft Outlook and create a new PST file.
    Open both the OST file and the new PST file side by side.
    Select the items you want to move from the OST file and drag them to the PST file.

Using Email Client Export Feature:
    If you have another email client like Thunderbird or Windows Live Mail configured with the same email account, you can try exporting the data from the OST file to these clients and then exporting them to PST format.

Using PowerShell Scripts:
    If you're comfortable with scripting, you can use PowerShell scripts available online to convert OST to PST. However, this method requires technical expertise and caution.

While these methods are free, they may not always be as reliable or efficient as dedicated third-party software solutions. Additionally, manual methods can sometimes lead to data loss or corruption if not executed properly. If your OST file is critical and contains important data, it's recommended to use a reliable third-party OST to PST converter tool like Kernel for OST to PST Converter for safer and more efficient conversion. https://www.nucleustechnologies.com/ost-to-pst-converter.html