Thank you all for your replies. As you noticed, I’m a bit amateur on watches and learning about.
I think the seller truly believe that it’s an original Omega.
If you think the seller is trying to be honest, I’d suggest mutually going to a jeweler/watchmaker you trust. That way the ‘bad’ news doesn’t have to come from you and in my experience, if they won’t agree to such a meet it wasn’t with good intentions to start. If it’s not a f2f, I’d suggest he/she takes it to a source you both agree on and ask for a pic of their review. Just my own method.