Casper wrote:
I am curious if anyone has put vinyl plank flooring over osb.

Recently we removed sheet vinyl from two rooms to find it was laid
over osb and not glued down. It was there for almost twenty years,
flat as a board and no issues. I was surprised it wasn't glued. Since
the subfloor seems like new, I am trying to decide if it really needs
any further underlayment.

Thoughts on this?

Contact the manufacturer and see what they have to say. Only their
response will be the one that matters.