Which is the correct saying when fasting: "Indeed, I am fasting" or "O Allah, indeed I am fasting," and what is the difference between them?

The correct approach is for the fasting person, when insulted or attacked, to say "Indeed, I am fasting" with their tongue, making their opponent hear it, so that the latter may be deterred. For absolute speech is that which is by the tongue, not by the heart. Adding the word "Allahumma" before "na'am" (yes) is permissible, as it is for seeking blessings, bearing witness by Allah, and affirming truthfulness.