To convert OST files to MSG format with attachments for free, you can use Microsoft Outlook. Open your OST file, select the emails, and drag them to a folder on your desktop to save them as MSG files with attachments. This method is free but can be time-consuming.
For a quicker solution, the MailConverterTools OST to PST Converter is a great option. It easily converts OST to MSG while preserving attachments supports bulk conversion, and is simple to use. The free demo version allows you to convert up to 50 emails per folder.