 | Nasturtium palustris | IE, VCH36,VCH39, Lough Neagh | | | 5/8/1885 | GALW |
 | Nasturtium sylvestre | IE, VCH6,VCH7,VCH11, Carrick-on-Suir | Robert Lloyd Praeger | National Museum Ireland Robert Lloyd Praeger | 23/6/1900 | GALW |
 | Nasturtium sylvestre | IE, VCH8 Co. Limerick, Lough Gur | Robert Lloyd Praeger | Science & Art Museum Dublin Robert Lloyd Praeger | 15/8/1900 | GALW |
 | Nitella gracilis | IE, VCH21 Co. Dublin, Glasnevin | Maura Josephine Patricia Scannell | Irish National Herbarium Glasnevin Dublin | 3/9/1980 | GALW |
 | Odontites verna subsp. serotina | IE, VCH38 Co. Down, Dundonald | John S Pate | Queens University Belfast | 6/1962 | GALW |
 | Odontites verna subsp. verna | IE, VCH20 Co. Wicklow, Magherabeg | Thomas Gerard Francis Curtis | Irish Biogeographical Society | 29/7/1978 | GALW |
 | Oenanthe crocata | IE, VCH20 Co. Wicklow, Ardmore Point | Micheline Joan Sheehy Skeffington | Irish Biogeographical Society | 29/7/1978 | GALW |
 | Oenanthe lachenalii | IE, VCH12 Co. Wexford, Castlebridge | Botanical Society of The British Isles | | 29/8/1977 | GALW |
 | Oenanthe lachenalii | IE, East Galway, Oranmore | Micheline Joan Sheehy Skeffington | | 25/8/1984 | GALW |
 | Oenanthe pimpinelloides | IE, VCH9 Co. Clare | L Donnell Micheline Joan Sheehy Skeffington | | 21/5/1988 | GALW |
 | Oenanthe pimpinelloides | IE, VCH9 Co. Clare, Miltown Malbay | E De Fay Micheline Joan Sheehy Skeffington | | 28/7/1984 | GALW |
 | Oenanthe pimpinelloides | IE, VCH9 Co. Clare, Miltown Malbay | N E Scott | | 1/6/1984 | GALW |
 | Oenothera biennis | IE, Ambiguous locality (IE), Ardmore | | | | GALW |
 | Ophioglossum vulgatum | | Enda Mooney | | 1/7/1986 | GALW |
 | Ophioglossum vulgatum | IE, VCH10 North Tipperary, Portumna | Robert Lloyd Praeger | Science & Art Museum Dublin Robert Lloyd Praeger | 24/6/1898 | GALW |
 | Ophioglossum vulgatum | IE, VCH13 Co. Carlow, Ballintemple | Robert Lloyd Praeger | Science & Art Museum Dublin Robert Lloyd Praeger | 4/6/1899 | GALW |
 | Ophioglossum vulgatum | IE, VCH18 Offaly, Lough Goura | Robert Lloyd Praeger | National Museum Ireland Robert Lloyd Praeger | 25/6/1898 | GALW |
 | Ophioglossum vulgatum | IE, VCH21 Co. Dublin, North Bull Island | Declan Doogue | Forest & Wildlife Service Irish Biogeographical Society | 7/1969 | GALW |
 | Ophioglossum vulgatum | IE, VCH25 Co. Roscommon, Lough Key | | | 5/1940 | GALW |
 | Ophioglossum vulgatum | IE, VCH28 Co. Sligo, Glencar | Robert Lloyd Praeger | Robert Lloyd Praeger | 7/1903 | GALW |
 | Ophioglossum vulgatum | IE, VCH39 Co. Antrim, Shanes Castle | | | 26/6/1891 | GALW |
 | Oreopteris limbosperma | IE, VCH11 Co. Kilkenny, Cappanagh | | | 1/8/1898 | GALW |
 | Oreopteris limbosperma | IE, VCH11 Co. Kilkenny, Thomastown | | | 1/8/1898 | GALW |
 | Oreopteris limbosperma | IE, VCH36 Tyrone, Coneywarren Glen | Mary Patricia Happer Kertland | Queens University Belfast | 6/1957 | GALW |
 | Origanum vulgare | VCH21, Lucan | | | 11/8/1894 | GALW |
 | Origanum vulgare | IE, VCH7 South Tipperary, Ardfinnan | Emer Ni Challarain | | 12/7/1965 | GALW |
 | Origanum vulgare | IE, VCH18 Offaly, Tullamore | | | 30/8/1899 | GALW |
 | Origanum vulgare | IE, VCH21 Co. Dublin, Templeogue | | | 28/7/1902 | GALW |
 | Origanum vulgare | IE, VCH34 East Donegal, Saints Island | Peter Hamilton Carvill | Irish Biogeographical Society | 8/1973 | GALW |
 | Origanum vulgare | IE, VCH37 Co. Armagh, Loughgall | W F Johnson | | 9/1894 | GALW |
 | Orobanche alba | | | Queens University Belfast | | GALW |
 | Orobanche minor | IE, VCH21 Co. Dublin, Rathcoole | | | 6/10/1931 | GALW |
 | Orobanche minor | IE, VCH21 Co. Dublin, Rathcoole | | | 3/10/1931 | GALW |
 | Orobanche minor | IE, VCH21 Co. Dublin, Rathcoole | | | 6/10/1931 | GALW |
 | Orobanche minor | IE, VCH21 Co. Dublin, Rogerstown | | | 6/1931 | GALW |
 | Osmunda regalis | IE, VCH15,VCH17,VCH25, Ballinasloe | M Aheane | | 6/1983 | GALW |
 | Osmunda regalis | IE, VCH1 South Kerry, Dunquin | Proinsias Ni Dhalaigh | | 21/7/1965 | GALW |
 | Osmunda regalis | IE, VCH11 Co. Kilkenny, Brandon Hill | Robert Lloyd Praeger | Science & Art Museum Dublin Robert Lloyd Praeger | 1/8/1898 | GALW |
 | Osmunda regalis | IE, VCH16 West Galway, Spiddal | Declan Doogue | Forest & Wildlife Service Irish Biogeographical Society | 8/1969 | GALW |
 | Osmunda regalis | IE, VCH27 West Mayo, Clare Island | | | 7/1903 | GALW |