What kind of Girls does Naotsugu like?
As you have guessed, Naotsugu was about to say おパンツ, or panties. However, he was kicked by Akatsuki (who went through the trouble of leaving the kitchen momentarily just to kick him) before he managed to finish his sentence.

This also matches the running gag where he would be kicked by Akatsuki, or when she is not around, he would have to dodge some natural phenomenon conveniently occurs before he manages to finish saying anything pervert. The first case frequently occurs in the first few episodes of the first season (among other occurences here and there), and the second case can be seen 8 minutes into episode 2 of the second season.