vape nerd, it'll come with the RM2, and you may or may not need to use it. You want your air hole in the RM2 to be facing away from you when you vape so that the air intake is not obstructed. When you screw the RM2 into the 510 connector, it may or may not face the way you need it to. The o-ring will help you adjust that placement so that it's facing out, or away from you when you vape. It goes down in the drip well around the connector, then you screw the atty in. I have a couple of REO/RM2s that I use it on, and a couple that I don't need it on. You'll see when you get it, but that's it. They're tiny (extra thin) and easily lost, so pay attention.
EDIT: Yours is coming tomorrow? You are in for a HUGE treat! The RM2 will take REO vaping to a whole new level. Be excited.