- O'NEIL, BRYAN HUGH ST. JOHN (1905 - 1954), archaeologist
- OAKELEY, Tanybwlch Maentwrog - see EVANS
- OGMORE, 1st Baron - see WILLIAMS, DAVID REES
- OLD PRICE - see PRICE, JOHN
- OLIVER, DAVID (fl. 1785-1814), Baptist minister
- OLIVER, EDWARD (1720 - 1777), early Methodist and Moravian, a carpenter
- OLIVER, JOHN (1838 - 1866), poet
- OLIVER(S), THOMAS (1725 - 1799), Wesleyan preacher
- OLIVER, THOMAS - see OLIVERS, THOMAS
- OLLIVANT, ALFRED (1798 - 1882), bishop
- ORMSBY, - see WYNN
- ORMSBY-GORE, - see WYNN
- ORMSBY-GORE, FANNY MARY KATHERINE - see BULKELEY-OWEN, FANNY MARY KATHERINE
- ORMSBY-GORE, WILLIAM DAVID (1918 - 1985), politician, diplomat, media impresario
- ORMSBY-GORE, WILLIAM GEORGE ARTHUR (1885 - 1964), politician and banker
- OSBWRN WYDDEL (fl. 1293), Irish nobleman and ancestor of landed families in Merioneth
- OSSIAN GWENT - see DAVIES, JOHN
- OUDOCEUS (fl. late 6th century), saint
- OULTON, WILFRID EWART (1911 - 1997), RAF officer
- OVERSTONE, Baron - see LOYD, LEWIS
- OWAIN ab EDWIN (d. 1105) Tegeingl, landowner
- OWAIN ALAW - see OWEN, JOHN
- OWAIN ap CADWGAN (d. 1116), prince of Powys
- OWAIN ap GRUFFYDD (d. 1236), prince of Deheubarth
- OWAIN ap GRUFFYDD ap GWENWYNWYN (d. 1293) - see GRUFFYDD ap GWENWYNWYN
- OWAIN ap GRUFFYDD (fl. 1260), prince of Gwynedd
- OWAIN ap LLYWELYN ap MOEL y PANTRI (fl. 15th century), poet - see LLYWELYN ap MOEL Y PANTRI
- OWAIN ap THOMAS ap RHODRI (d. 1378), soldier of fortune and pretender to the principality of Wales
- OWAIN BROGYNTYN (fl. 1160-1188), prince of Powys
- OWAIN CYFEILIOG (c. 1130 - 1197), prince and poet
- OWAIN FYCHAN ap MADOG ap MAREDUDD (d. 1187), prince of Powys
- OWAIN GLYNDWR (c. 1354 - 1416), 'Prince of Wales'
- OWAIN GOCH - see OWAIN ap GRUFFYDD
- OWAIN GWYNEDD (OWAIN GWYNEDD; c. 1100 - 1170), king of Gwynedd
- OWAIN GWYNEDD (fl. c. 1550-1590), poet
- OWAIN LAWGOCH - see OWAIN ap THOMAS ap RHODRI
- OWAIN MÔN - see JONES, EDWARD
- OWAIN MYFYR - see JONES, OWEN
- OWAIN TUDOR (c. 1400 - 1461), courtier
- OWAIN, Syr DAFYDD, cleric and poet
- OWAIN, Sir DAVID - see OWAIN, Sir DAFYDD
- OWAIN, OWAIN LLEWELYN (1877 - 1956), litterateur, musician and journalist
- OWEN family Cefn-hafodau, Glangynwydd, Glansevern, Llangurig
- OWEN family Bodeon, Bodowen,
- OWEN family Plas-du,
- OWEN family Peniarth,
- OWEN family Orielton,
- OWEN, BARON - see OWEN, LEWIS
- OWEN, - see LEWIS
- OWEN, Llwyn-du Llangelynnin - see LEWIS