Yup. Just be completely and totally straightforward. There's no other way to clear this up. It'll be hard, but it'll be much harder to untangle things in the long run if you don't take that path.
And seriously, an OPP for one of your partners and not another is just kind of weird? What is the deal with that?
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.