Answer: bourbon
I always thought this scene might have included a bit of an ad-libbed-over blooper. Paul replies "bourbon", and then O.J. asks if he wants it with rocks. Paul says, looking distracted, "Yeah. No! Water." O.J.'s response is, "Okay, but rocks first, right?", and Paul, slightly confused, snaps back to the conversation and agrees. Something about the way the lines were delivered made me wonder if George Peppard didn't goof a bit and Martin Balsam was trying to get him back on track. They proceeded neatly from there, however, so maybe it really was scripted that way. I just thought it seemed a little less polished than the rest of the film's dialogue, so it stood out to me.