Serpas need extensive training in use - does the open carry civilian do that? There are very unfortunate accidents with these under stress which may or may not be prevented by training. We've had two shoot themselves with such in local matches.
Thus, strapping on a Serpa without much, much practice (and does Oscar Open-Carry do that) is a tad risky.
That's not all. John Farnam doesn't allow them in his courses. He cited incidents where the mechanism was jammed up by a twig. I believe they occurred in the more rigorous envirnonment of combat zones.