There are 36 unique child surnames beginning with O:

O'Brian	O'Brien	O'Conner	O'Donnel	O'Harra	O'Laughlin	Oberdier
Oda	Odea	Oehler	Ogden	Oglesby	Ohara	Oids
Olds	Olive	Oliver	Oller	Onsler	Opphile	Orabaugh
Orahood	Orders	Organ	Ormeroid	Orr	Orrison	Orsbern
Osborn	Osborns	Otte	Otto	Overacker	Overfield	Overholser
Owen

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