Houses in bold are in Mark Bence-Jones 

42 O’Connell Street, Dublin 

O’Harabrook, County Antrim https://aguidetoirishcountryhouses.com/2024/11/06/oharabrook-ballymoney-county-antrim/

Oak Park, Carlow – agricultural college https://aguidetoirishcountryhouses.com/2024/12/13/oak-park-painestown-co-carlow/

Oak Park, or Collis-Sandes House Tralee, Co Kerry https://aguidetoirishcountryhouses.com/2025/02/15/oak-park-or-collis-sandes-house-tralee-co-kerry/

Oakfield Park, Co Donegal – gardens Section 482  https://aguidetoirishcountryhouses.com/2022/11/03/oakfield-park-oakfield-demesne-raphoe-co-donegal-garden-only/

Oakgrove, Killinardrish, Co Cork  http://aguidetoirishcountryhouses.com/2025/01/05/oakgrove-killinardrish-co-cork/

Oaklands, Co Wexford – lost 

Oaklands, Clonmel, Co Tipperary 

Oakley Park, Celbridge, Co Kildare (now St. Raphael’s) https://aguidetoirishcountryhouses.com/2025/02/17/oakley-park-celbridge-co-kildare-now-st-raphaels/

Oakley Park, Kells, Co Meath https://aguidetoirishcountryhouses.com/2025/06/05/oakley-park-kells-co-meath/

Oakport, Boyle, Co Roscommon https://aguidetoirishcountryhouses.com/2025/09/03/oakport-boyle-co-roscommon/

The Oaks, County Derry 

Oatlands House, Co Meath 

Odder Castle, Co Meath 

Odelville House. Ballingarry, Co Limerick – section 482 https://aguidetoirishcountryhouses.com/2024/10/16/odellville-house-ballingarry-co-limerick/

The Odeon (formerly the Old Harcourt Street Railway Station), 57 Harcourt Street, Dublin 2 – section 482 https://aguidetoirishcountryhouses.com/2024/10/23/the-odeon-formerly-the-old-harcourt-street-railway-station-57-harcourt-street-dublin-2/

Old Castle House, Dungar, Roscrea, Co Tipperary https://aguidetoirishcountryhouses.com/2025/08/24/old-castle-house-dungar-roscrea-co-tipperary/

Old Coastguard Station, Rosmoney, Westport, Co Mayo – section 482 

Old Conna Hill, Shankill, Co Dublin https://aguidetoirishcountryhouses.com/2025/01/15/old-conna-hill-shankill-co-dublin/

Old Connaught House, Shankill, Co Dublin – apartments https://aguidetoirishcountryhouses.com/2025/01/15/old-connaught-house-shankill-co-dublin-apartments/

Old Court, Douglas, Co Cork (not Old Court Castle also known as Kanturk Castle)  http://aguidetoirishcountryhouses.com/2025/01/05/old-court-douglas-co-cork/

Old Court House, Strangford, County Down http://aguidetoirishcountryhouses.com/2025/01/12/old-court-house-county-down/

Old Court, Strangford, County Down

Old Dromore, Mallow, Co Cork  http://aguidetoirishcountryhouses.com/2025/01/05/old-dromore-mallow-co-cork/

Old Glebe, Main Street, Newcastle, Co. Dublin – section 482 https://aguidetoirishcountryhouses.com/2019/12/31/the-old-glebe-newcastle-lyons-county-dublin/

Old Head, Kinsale, Co Cork  http://aguidetoirishcountryhouses.com/2025/01/05/old-head-kinsale-co-cork/

Old Mellifont Abbey, Tullyallen, Drogheda, County Louth – Office of Public Works https://aguidetoirishcountryhouses.com/2024/10/11/old-mellifont-abbey-tullyallen-drogheda-county-louth-office-of-public-works/

Old Rectory, Killedmond, Borris, Co Carlow  – section 482  https://aguidetoirishcountryhouses.com/2020/07/16/the-old-rectory-killedmond-borris-co-carlow/

The Old Rectory, Kilkenny Rd, Carlow, Co. Carlow https://aguidetoirishcountryhouses.com/2025/06/24/the-old-rectory-kilkenny-rd-carlow-co-carlow/

The Old Rectory, 17 Seafield Road West, Dublin 3, D03P793 https://aguidetoirishcountryhouses.com/2024/12/07/the-old-rectory-17-seafield-road-west-dublin-3-d03p793/

Old Rectory, Rathkeale, Co Limerick – section 482 https://aguidetoirishcountryhouses.com/2024/10/16/the-old-rectory-rathkeale-co-limerick-section-482/

The Old Rectory, Termonfeckin, Co. Louth https://aguidetoirishcountryhouses.com/2025/06/11/the-old-rectory-termonfeckin-co-louth/

Oldbridge, near Slane, Co Meath https://aguidetoirishcountryhouses.com/2024/10/11/oldbridge-house-county-meath-battle-of-the-boyne-visitor-centre/

Oldtown, Naas, Co Kildare https://aguidetoirishcountryhouses.com/2025/02/17/oldtown-naas-co-kildare/

Orangefield House, County Down http://aguidetoirishcountryhouses.com/2025/01/12/orangefield-house-county-down/

Orangefield Park, County Down see  http://lordbelmontinnorthernireland.blogspot.com/2017/06/the-blakiston-baronets.html

Oranmore Castle, Oranmore, Co Galway – section 482 in 2019, accommodation https://aguidetoirishcountryhouses.com/2023/04/13/oranmore-castle-oranmore-co-galway/

Oranmore Lodge Hotel, see Thorn Park, Oranmore, Co Galway https://aguidetoirishcountryhouses.com/2024/10/15/oranmore-lodge-hotel-previously-thorn-park-oranmore-co-galway-2/

Oriel Temple (see Collon House), Collon, Co Louth 

Orlagh House, Dublin – whole house rental https://aguidetoirishcountryhouses.com/2024/10/09/orlagh-house-dublin-whole-house-rental/

Ormeau, County Down – demolished http://aguidetoirishcountryhouses.com/2025/01/12/ormeau-county-down-demolished/

Ormonde Castle, Carrick-on-Suir, County Tipperary https://aguidetoirishcountryhouses.com/2024/09/05/ormond-castle-carrick-on-suir-county-tipperary-an-opw-property/

Osberstown House, Oberstown, Naas, Co Kildare https://aguidetoirishcountryhouses.com/2025/02/17/osberstown-house-oberstown-naas-co-kildare/

Oscar Wilde House, 1 Merrion Square, Dublin https://aguidetoirishcountryhouses.com/2024/10/13/oscar-wilde-house-merrion-square-dublin/

Ounavarra, Courtown Harbour, Co Wexford https://aguidetoirishcountryhouses.com/2025/06/05/ounavarra-courtown-harbour-co-wexford/

Owendoon House, now Jampa Ling Buddhist Centre, Cavan 

Owenmore House, Crossmolina, Mayo https://aguidetoirishcountryhouses.com/2024/10/17/owenmore-garranard-ballina-co-mayo/

Ozier Hill, Taghmon, County Wexford https://aguidetoirishcountryhouses.com/2025/08/31/ozier-hill-taghmon-county-wexford/