oh, for some reason I was thinking he was an hour away.. I guess I got mixed up with someone else's sitch.
If you tell OM, he is going to tell your W, so before you say anything to OM or his W, you need to enforce the boundary of not being willing to be in an open M with your WW. If she finds out you know and have done nothing about it, she will know she can walk all over you and you will do nothing. Not a very attractive trait, you NEED to address it with your WW first.
I think I would still follow my outline, I don't think you owe it to OM to give him a chance, I think you either need to expose the A or not. If you tell your WW and OM you know, they will agree to stop, and then go further underground so you don't know it continues. If it was me, I'd follow my plan outlined previously and let everyone take their own journey, I think it would be your best bet for your WW to work her way back to you.
Remember, this is a long journey, no matter what happens after you expose (if you do), this is not over and you are not going to magically return to happily married. You and your WW need to work on yourselves and only then, start to work on MR, if you don't, you will be back here within a few years.
M - 9 1/2 years 5/5/16 - Bomb drop - 3 week EA 10/31/16 - We sold house 01/10/18 - D Finalized