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- Oakes, Oaks, Oates, Oakkes, Oats, Oatt, Oak
- Oakland, Oaklan, Oaklin
- Oakley, Oalky, Oaklie
- Oakwood, Oakwoods, Oakkwood
- Obama, Obamas, Obamal
- Abando, Obando, Obandeo
- O’Baoghill, Boyle, Boile
- Oberly, Eberle, Eberly, Epperly, Oberle, Oberlin
- Obert, Oburt, Obertt
- Oberthier, Oberdeer, Oberdier, Overdeer
- O’Beirne, O’Burne O’Berne
- O’Berg, O’Burg, OBert
- Oberlin, Oberline, Oberlind
- O’Berst, Obar, Upper
- O’Berti, Uberto, O’Bert
- O’Boyle. Boyle, Boile
- O’Bradaigh, Brady, Bradey
- O’Brdain, Braden, Bradan
- O’Broin, Breen, Brean
- OBraonain, Brennan, Brennon
- O’Briain, O’Brien, O’Brien, Brian, O’Brian
- O’Broin, O’Byrne, O’Broyn
- O’Buachalla, Buckley, Buckly
- O’Byrne, Bran, Bron
- Ochoa, Wolf, Wolfe, Otxoa
- O’Caiside, Cassidy, Cassedy
- O’Callaghan, Callahan, Callaham
- O’Caoimh, Keefe, Keefer
- O’Caoimhain, Cavan Cavane
- O’Caoindealbhain, Quinlan, Quinlon
- O’Caolaidhe, Keeley
- O’Cathain, Cathan, Cathen
- O’Cathalain, Callan, Callon
- O’Cathasaigh, Casey, Casie
- O’Ceallachain, Callahan, Callihan
- Ocean, O’Sean, Ocen
- O’Ceallaigh, Kelly, Kelley
- O’Cearmada, Carmody, Carmady
- O’Ceithearnaigh, Kearney, Kearny
- O’Ciardha, Carey, Carrey
- O’Cinneidigh, Kennedy, Kenndy
- O’Clerigh, Clerke, Clarke
- O’Clery, O’Clerigh, Clerke
- O’Cnaimhin, Cnamh, Bone
- O’Cochlain, Cochllan, Cape, Hood
- O’Coigligh, Quigley, Quigly
- O’Coileain, Coilean, Coileane
- O’Camhraidhe, Camhraidhe,
- O’Conaill, Connell, Connall
- O’Conaire, Conroy, Conray
- O’Conchobair, O’Connor, Connor
- O’Conghalaigh, Connolly, Connally
- O’Connell, Connell, Connall
- O’Connor, O’Conchobhair, Conchobar
- O’Corcrain, Corcoran, Corocoran
- O’Corra, Corra, Spear
- O’Corraidhin, Curran, Curron
- O’Cuaig, Quigg, Quig
- O’Cuana, Cooney, Couney
- O’Cuidighthigh, Cuidightheach
- O’Cuilinn, Cuileann, Holly
- O’Cuinn, Quinn, Quin
- O’Cuinneagain, Cunningham, Cunningam
- O’Cuirc, Quirke, Quirk
- O’Dalaigh, Daly, Daley
- O’Damhain, Damhan Damham
- O’Deasmhumhnaigh, Desmond, Dasmond
- Odell, Odel, O’dell, Odle, Wad, Hill
- Odenton, Odenston, Odentan, Odenson
- Odenwelder, Oddenwelder, Odenwilder
- O’Deoradhainn Doran, Dorin
- O’Doimasaigh, Dempsey, Dempsy
- O’Dochartaigh, Doherty, Dorharty
- O’Doibhilin, Devlin, Devlan
- O’Doirnain, Durnin, Duren
- Odom, Odon, Odomn, Odum
- O’Domhnaill, O’Donnell, O’Donnall
- O’Donnchadha, Donnoghue, Donnohue
- O’Donndubhain, Donovan, Donavan
- O’Donnell, ODomhmaill, Domhnall
- O’Donnghaile, Donnelly, Donnelley
- O’Doyle, Doyle, Doile, O’Dubhghaill
- O’Dubhain, Duane, Dwane
- O’Dubhshlain, Dolan, Dollan
- O’Dubhshlaine, Delaney, Delany
- O’Duhthaigh, Duffy, Duffey, Dark
- O’Duibh, Dark, Dubh
- Oelberg, Oil Hill, Berg, Mountain, Hill
- O’Fallamhain, Fallon, Fallan
- O’Faolain, Whelan, Whelon
- O’Fearghail, Farrell, Farrall
- Offermans, Offerman, Donation
- O’Fioagain, Finnegan, Finnigan
- O’Fionnain, Fannon, Fannan
- O’Flannagain, Flanagan, Flanigan
- O’Flannghaile, Flannery, Flannary
- O’Floinn, Flynn, Flinn
- O’Foghladha, Foley, Folley
- O’Frighil, Friel, Freil
- O’Gallchobhair, Gallagher, Gallager
- Ogawa, Small, River, Stream
- Ogden, Oggden, Ogten, Oak, Valley
- Ogilvie, Ogilvy, Ogilbie
- Ognianov, Ognyanov, Ognyenov, Ognyan
- O’Gormain, Gorman, Gorrman, O’Gorman
- O’Grada, Grady, Gradie
- Ogtrop, Ochtrup, Town in Germany
- Oh, Wu, Korean Form
- O’Hanegan, O’Hannagain, Hannagan, O’Hannigain, OHannegan
- O’Haodha, Hayes, Hays
- O’Haonghuis, O’Hennessy, Hennessy, Hennessee
- O’Heaghra, O”Hara, Hara
- O’Heachthughearna, Echthigern
- O’Hearn, O’Heachthighearna, O’Herns
- O’Heideain, Eidean, Clothes
- O’Heideain, Hines, Hynes
- O’Heidin, Clothes, Armour
- O’Heidirsceoil, Driscoll, Driscall
- O’Hicidhe, Hickey, Hicky
- O’Hlfearnain, Heffernan, Heferman
- O’Hogain, O’Hogan, Hogan
- O’Huiginn, O’Higgins, Higgins
- Oilfield, Oilfeild, Oilfeld
- Ojeda, Ojeda River, Folia, Leaves
- Okabe, Hill, Ridge
- Okamoto, Root, Base
- Okamura, Hill. Ridge of Town or Village
- O’Kane, O’Cathain
- Okazaki, Hill or Ridge of Cape or Peninsula
- O’Keefe, Keefe, Keef
- O’Laoghaire, O’Leary, O’Learie
- O’Leannain, O’Lennon, Lennon
- O’Leastro, Wild Olive
- Ole, Son of Ole
- Olin, Ollin, Olen
- Oliveira, From Oliveria in Portugal
- Oliver, Olivier, Olliver
- Oliversen, Oliverson, Oliversson, Olivier
- Olmstead, Olmsted, Olmsteade
- Olney, Olny, Ollney
- Olofsdotter, Olofsdatter, Daughter of Olof
- Olofsson, Son of Olof
- Olson, Ollson, Olsen, Son of Ole
- O’Luain, O’Lane, Lane
- O’Madaidhin, Dog, Mastiff
- O’Malley, O’Maille, Malley
- O’Manachain, O’Monahan, Monahan
- O’Maolagain, O’Milligan, Milligan
- O’Maolain, O’Mullen, Mullen
- O’Maonaigh, O’Mooney, Mooney
- O’Marcaigh, O’Markey, Markey
- O’Mathghamhna, O’Mahoney, Mahoney
- O’Meadhra, O’Meara, Meara
- O’Mochain, O’Mohan, Mohan
- O’Muircheartach, O’Moriarty, Moriarty
- O’Muireadhaugh, O’Murray, Murray
- O’Murchadha, O’Murphy, Murphy
- Onderdonk, Onderdonck, Underdonk
- Ongaro, Ungaro, Ongaroe
- O’Neal, O’Neale, O’Neil, O’Neill, O’Niall
- O’Nuallain, O’Nolan, Nolan
- Oorschot, VanOirschot, Oirschot
- Op Den Dyke, Updyke, UpDyke, UpDike
- Opeyemi,Opeyemie, Oppeyemi
- Oppenhiemer, Oppenhiem, Openheim
- O’Proinntigh, Prunty, Proinntigh
- Opie, Oppie, Opey
- Oquendo, Okondo, Oquendoe
- O’Quinn, O’Quin, O’Quinne
- O’Rabhartaigh, O’Rafferty, Rafferty
- O’Raghailligh, Raghailligh, Raghailigh
- Orange, Orangge, Orang
- Orban, Orrban, Orbane
- Orcutt, Orcut, Orrcut
- Ordway, Ordly, Ordley
- O’Reilly, Reilly, Reily
- Orellana, Aureliana, Aurelius
- Orendorph, Orendorff, Orndorff, Ohrendorph, Orndoph
- Orenstein, Orenstine, Hornstone
- O’Riagain, O’Reagan, Reagan
- O’Rian, Oriain, Rian
- O’Rinn, Rinn, Star
- O’Rioghardain, O’Riordan, Riordan
- Oriol. Orriol, Orioll
- Orlando, Orelando, Orlandoe
- Orlov, Oryal, Eagle
- Orman, O’Ruaidh, Ruaidh, Ormond, Ormonde
- Orne, Orrne, Orn
- O’Rodagh, Rodagh, Rodach
- Orosz, Orroz, Oross
- Orozco, Wisdom, Bringer of Wisdom
- Orr, Ore, Or, Oar, Orre, Oher
- Orsini, Ursus, Lettlebear
- Ortega, From Ortega in Spain, Ortiga
- Orth, Orthe, Oarth
- Ortiz, Orti, Son of Orti
- O’Ruadhain, Ruadhain, Ruadhan
- Orville, Orvill, Orvil
- Orwell, Orwelle, Orwel
- Osborn, Osborne, Osbourn, Osbourne, Osburn, Ozburn, Ozborne, Ozborn, Ausburn
- O’scolaidhe, Scolaidhe, Scholaidhe
- O’Seachasaigh, O’Shaughnessy, Shaughnessy
- O’Seaghdha, O’Shea, Shea
- O’Seanain, Seanain, Shannon
- O’Seighin, Seighin, Seigene
- O’Shields, Ocheals, Osheals
- Osgood, Osgoode, Ozgood
- Oshiro, Oshero, Castle
- O’Sioda, Sioda, Silk
- O’Sirideain, O’Sheridan, Sheridan
- O’Sluaghadhain, O’Sloan, Sloan
- Osmonov, Osmon, Son of Osmon
- Ostberg, Eastberg, Mountain
- Oster, East, Eastside
- Ostergaard, Ostergard, Enclosed
- Osterhout, Osterhouht, Osterhoudt, Oosterhoudt,
- Ostrander, Ostranter, Ostander
- Ostrom, Ostroms, Ostrome
- Ostrowski, Ostrosky, Ostro, Riverisland
- Oswald, Oswold, Osswald, Oswalt
- O’suaird, Suaird, Suart, Sigurd
- O’Suileabhain, O’Sullivan, Sullivan
- O’Tadhgain, O’Teagan, Teagan
- O’Taidhg, O’Teague, Teague
- O’Teachain, O’Teahan, Teahan
- Otero, Ottero, Hill
- O’Tighearnaigh, OTierney, Tierney
- Otis, Ottis, Son of Ode
- O’Toole, Toole Toule, Tuathal, O’Tool
- O’Treasaigh, O’Tracey, Tracey
- Ottinger, Ottingner, Otinger
- Ott, Otto, Otten, Ottosen
- Otty, Otte, Othy
- Outterridge, Uhtric, Oterridge
- Otxoa, Ottxoa, Wolf
- Oursler, Ursel, Oberursel
- Outlaw, Offlaw, Outlaws
- Overfield, Overfeld, Overfelt, Overfields
- Overholtzer, Overholt, Oberholtzer
- Overmyer, Obermeyer, Obermier, Overmeier, Overmier
- Overstreet, Overstreets, Oberstreet
- Overturf, Overtauf, Overterf
- Ovesen, Ove, Son of Ove
- Owings, Ouings, Oweings
- Owens, Owen, Owan, Owings, Owing. Owain
- Owston, Ouston, Owsten
- Oxendine, Oxendin, Ozendine
- Oxford, Oxforde, Oxfford
- Oxon, Oxan, Oxin
- Oyler, Eyler, Oiler
- Ozols, Ozolins, Ozolinsh
- Ozwald, Ozwalt, Oswalt, Oswald, Ozwall