Two toned readings, both stand-alone sentence-initial cries:
哦 (rising) signals doubt or surprise - 'oh? really?' - the pitch climbs as if to question what was just heard;
哦 (falling) signals a just-clicked realization - 'ah, I see' - pitch drops as the penny lands. Same character; pitch contour does all the emotional work. A rare archaic reading
哦 means 'to chant / recite verse' - literary only, you'll meet it in classical poetry but not in conversation.
