哦 (ò): oh (one has just learned something)

Oh, so you were sick yesterday—no wonder you didn't come to class.

Oh, I see—you mean I have to press this button twice to save, right?

Oh, no wonder I couldn't find my keys—they were in my pocket all along.

Oh, that's why this restaurant is so busy on weekends—many people come for brunch.

Oh, I just noticed—your new glasses look great.