"oh okay" is probably about the best thing you could say. Don't treat it like a big deal. I can say that the single biggest thing I appreciate when talking to someone, is that when I look back they're still paying attention. I almost always break eye contact when stuttering, but when I look back I notice that most people are trying to 'get away'. They're studying their coffee mug, they've suddenly noticed that patch of wall is fascinating, or whatever. But it just means so much to see that they're not doing that, they're just listening to me like I'm not a normal person.