We use “one of” when we want to say something is one part of many. “One-off” means that something happens one time.
You can use “one of” when you want to say that one item is one of a group.
She is one of eight children.
She is one of my friends who plays tennis.
In the second example above the subject of the sentence is “she” so we need to conjugate “does” with “she”.
A “One-off” event is something that happens one time and is never to be repeated.
That was a one-off parade! You will never see anything like that again!
“One-off” functions as a noun and doesn’t really have a plural as it only happens once!
Once-off or One-off?
“Once-off” is perfectly acceptable in place of “one-off”.