I had this large PST (OST would be same) and all I did was sort on the messages by size and the top ten were the offenders and I deleted the ones that could be deleted and the others were looked and and found to have large pictures or PDFs so I edited the message to remove or backup the picture or PDF.
No app was needed and no new Outlook skills were learned.
I am working Outlook from 4years back, but recently I am facing a problem of too large OST files. Now, Outlook is showing the error of OST file too large. I want a reliable solution to reduce the size of the OST file. Because the emails are important for me I don't want any data loss. My operating system is Windows 10 and using Outlook 2010 version.