What is your current Orange cab that you are looking to add to? The 15? The 410?
Most everyone on here will advise against mixing driver sizes, and will advise just getting a second version of what you currently are playing if you want to add speakers. So if you're playing the OBC115, get a second (and same for the 410).
I can speak to mixing the OBC drivers, as I own a 115 and a 410 and have played both cabs on a few occasions. For all intents and purpose, the guys on the forums are correct about the negatives in doing so.
That said, if you're looking at trying to get one of the 210's, which as you mentioned (and I think you are correct) are not even made anymore, the prices I'm seeing for that cab are just shy of what you could purchase a new OBC410 for. I would think that should be a deterrent enough for you to look elsewhere.