She knows better. She knows from speaking with Nunnally that Lelouch isn't a totally evil schmuck that is bent on world domination (in before Suzaku die-hards start clamoring he is). If the spoilers are to be believed, she will think about leaving him but decide against it.
From Oghi's point of view, Lelouch TOOK OUR DERBS. He rolled in to town and suddenly Oghi wasn't the top dog anymore. Once he knows about geass and that Zero is a discarded Britannian prince, the logical conclusion is "Oh, that guy must have used his mind control power on us to make us fight for him".
Of course, that theory doesn't really hold up if you analyze it. But they're Japanese. Their inferiority complex will activate and then they will start to hate the gaijin. And of course, Britannia has been their enemy this entire time. So he is not to be trusted.