If we want to be very picky it is "open source" not "Open Source".

The capitalised term indicates it's a license approved by the Open Source Initiative, which is commonly used as the basis of "actual open source"

(I do agree that this gets a little murky, especially with folks who don't understand the nuance, which is a large percentage of folks talking about this)

I do agree that not calling something even "open source" if it's under a license like the FSL is best