I recommend using this MacProTools OST to PST Converter for Mac, which makes it easy to convert several OST files into the EMLX format at once. Additionally, it converts all OST email attachments to EMLX format. Without any restrictions, it can convert OST files of any size into the EMLX format.
Read more: Converts OST files to EML format.
Read more: Converts OST files to EML format.