A free method to convert an OST file to PST format is using Microsoft Outlook's built-in export wizard if your original Exchange account profile is still active. But if your OST files are corrupted or inaccessible, you need to use some professional tools like Softaken OST Converter, WholeClear OST to PST Converter or any reputable tool. These tools are designed for converting OST files to PST format without requiring Outlook or an active Exchange connection.