O benim kızım = She is "MY" daughter
O kızım = She is my daughter
But i think you dont use "O kızım" because if you want to express that she is your daughter you should use benim word. For example while you are signing your daughter among other children you should say "Şu benim kızım" so you will need to use benim. But if you ask what is "o kızım" the answer is "she is my daughter.
Kızım : My daughter
You use that while you are talking to your daughter. As she call you "baba" you call her "kızım"