What is OLK15message files and how to recover it?
Apart from OLM files, Mac Outlook creates temporary files in the backend called as OLK file. These contain data exclusively related to individual profiles. It is stored in the identity folder of the respective profile to which it relates. The OLK file only stores the header information of the mails data. For storing message content without attachment Mac Outlook Creates OLK14/15MSGSOURCE. 

The location of OLK Files:
[user]/Documents/Microsoft User Data/Office 2011 Identities/Main Identity/Data Records/Messages Source

Its recovery is a bit risky one but here's an article that explains how to recover OLK15message files: http://olk15messagerecovery.blogspot.com/