Booting from MOS 2.5 CD worked fine.

I did more search and found another Mac Mini user reporting the same problem here, but for previous version:
https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=7131&forum=3#73598

Followed the instructions and it worked out fine. It turns out the syntax I used to boot from OF was incorrect. It should read:

boot cd:,\\boot.img

Thanks again for everyone's help... and the lesson is....
do your search first, then shoot from the hip

BTW, the update worked perfectly.