- O Hasten
- O Isham
- O'brien
- O'callaghan
- O'connell
- O'connor
- O'donnell
- O'dwyer
- O'farrell
- O'hara
- O'keefe
- O'leary
- O'mahony
- O'neill
- O'reilly
- O'rourke
- O'sullivan
- O'toole
- Oakes
- Oakleaf
- Oaks
- Oates
- Oatis
- Obajtek
- Obanion
- Obara
- Obeda
- Obenshain
- Oberfield
- Oberlin
- Oberman
- Oberry
- Oberski
- Oberto
- Oberwise
- Obeytek
- Oblander
- Oblaza
- Obniski
- Oboikovitz
- Oboyle
- Obrein
- Obrien
- Obrien-Bremer
- Obrochta
- Obuchowski
- Obyrne
- Ocampo
- Ocampos
- Ocegueda
- Ochal
- Ochenkowski
- Ochoa
- Ochodnicky
- Ochs
- Ochwat
- Ocoa
- Oconnell
- Oconnor
- Oconor
- Odea
- Odeal
- Odei
- Odekirk
- Odell
- Oden
- Oder
- Odeshoo
- Odish
- Odishoo
- Odom
- Odoms
- Odonell
- Odonoghue
- Odonovan
- Odowd
- Odyniec
- Oeder
- Oehman
- Oehmen
- Oehrke
- Oelrich
- Oermann
- Oest
- Oeste
- Offenbacher
- Offenberg
- Offutt
- Oflaherty
- Oflynn
- Ogara
- Ogasawara
- Ogawa
- Oglesby
- Ogrady
- Ogrodnik
- Ogrodny
- Ohagan
- Ohlhuausen
- Ohlson
- Ohm
- Ohms
- Ohren
- Ohringer
- Oishi
- Ojeda
- Ojermark
- Okada
- Okafor
- Okal
- Okane
- Okelly
- Okey
- Okic
- Okon
- Okoro
- Okrezesik
- Okrongley
- Okrzesik
- Okumura
- Okun
- Okunowski
- Okura
- Okwu
- Olali
- Olander
- Olaughlin
- Olave
- Olazaba
- Olczak
- Oldenburg
- Oldham
- Olds
- Olea
- Olear
- Oleari
- Olechno
- Olechnowicz
- Oleck
- Olejniczak
- Oleksiak
- Oleksy
- Oleszczak
- Oleszkiewicz
- Oleynikov
- Olguin
- Olijnyk
- Olin
- Olinger
- Oliva
- Olivares
- Olive
- Oliver
- Olivera
- Oliveros
- Olivet
- Olivier
- Olivo
- Olker
- Olle
- Ollestad
- Ollie
- Olma
- Olmos
- Olortegui
- Oloughlin
- Olsen
- Olsinski
- Olsman
- Olsofka
- Olson
- Olsowski
- Olsson
- Olswang
- Olszak
- Olszewski
- Olszowka
- Oltmanns
- Oluwole
- Olvera
- Omalley
- Oman
- Omer
- Omiatek
- Omotade
- Onate
- Ondas
- Oneal
- Oneil
- Oneill
- Onesto
- Ong
- Onik
- Onoda
- Ontiveros
- Onwuli
- Onyeali
- Opacian
- Opalinski
- Opasinski
- Opelar
- Openlander
- Openshaw
- Opitz
- Opolinski
- Opon
- Opozda
- Oppenheimer
- Oquendo
- Oquinn
- Orantes
- Orbach
- Orban
- Orca
- Ordaz
- Ordiales
- Ordman
- Ordower
- Ordyna
- Oregan
- Oregel
- Orellana
- Orenic
- Orenstein
- Oria
- Oriley
- Orlandi
- Orlando
- Orlich
- Orlikoff
- Orlinsky
- Orlow
- Orluck
- Ornelas
- Ornowski
- Orona
- Oropeza
- Orozco
- Orphan
- Orr
- Orre
- Orrico
- Orsello
- Orseske
- Orta
- Ortega
- Ortega-Rico
- Ortenberg
- Ortiz
- Orton
- Orusa
- Orybkiewicz
- Orzech
- Orzechowski
- Orzel
- Osada
- Osberger
- Osborne
- Osbourne
- Osby
- Oseguera
- Osei-Kuffuor
- Osepek
- Oshana
- Oshea
- Oshinski
- Osier
- Osika
- Osinowo
- Osinski
- Oskvarek
- Osley
- Osmanski
- Osmund
- Osness
- Osoba
- Osolin
- Osorio
- Ospina
- Ossanna
- Ostapowicz
- Oster
- Osterman
- Ostermann
- Ostian
- Ostrander
- Ostrega
- Ostroff
- Ostromogilskaya
- Ostrov
- Ostrovskaya
- Ostrovskiy
- Ostrovsky
- Ostrowski
- Osuley
- Osullivan
- Osusky
- Oswald
- Otero
- Otley
- Ott
- Ottens
- Otterson
- Otto
- Ottoman
- Otvos
- Ou
- Ouimet
- Ousley
- Outlaw
- Outten
- Ouyang
- Ovalle
- Overby
- Overland
- Overman
- Oversmith
- Overton
- Oveson
- Oware
- Owcarz
- Owczarek
- Owczarski
- Owen
- Owens
- Owiredu
- Owusu
- Oya
- Oyervides
- Ozag
- Ozaki
- Ozanic
- Ozga
- Oziel
- Ozimek
- Ozmina
- Ozog
- Ozolins