You should be able to export single e-mail message out of opened OLM file to a common EML file. Might only work in Mac version of Outlook.
Convert Outlook for Mac e-mail database to Email message.
We have found one software suitable in our database for this conversion.
Apple macOS / Mac OS X software - convert olm to eml on OS X
Mac version of the popular e-mail client and personal information manager from Microsoft
Convert from olm file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.