I have an interest in stick rockets, and have tried lots of ways to model these and other 'odd' rockets in OR, and other Barrowma-based tools, and frankly, it just isn't up to the task.
Barrowman (used by OR) assumes that body tubes make no contribution whatsoever to CP; The CP is determined by a geometric average of the CP of the nosecone, fins, and whatever transitions you have.
So, for example, if your rocket is just a NC and a body tube, it makes no difference how long your body tube is - the CP will always be way up in the nose.
If you just have body tube and fins, then again nothing you do with the BT makes any difference - the CP will just be the CP of the fins. In fact, even changing something like the width of the fins won't have any effect, because the width of the fin has no effect on the Cp of the fin itself.
In the ORK design above, I think that what has happened is that OR sees two body tubes, places the CP of each in the center, and then uses some geometric average of these center points just to avoid a divide-by-zero error. You'll note that changing the diameter of either tube has absolutely no effect on the Cp of the rocket, which is counterintuitive.
So, I think that OpenRocket, and Barrowman (as originally formulated) is worthless for determining the Cp of a stick rocket. You might have gotten a stable design in OR, and a stable rocket in real life, but this is probably a pure coincidence and should not be counted upon.
I think the cardboard cutout method is probably far more reliable for stick rockets than Barrowman.