O'Callaghan
- Annals of the Four Masters
- Armagh
- Athboy
- Baron Inchiquin
- Barry O'Callaghan
- Bill O'Callaghan
- Billy O'Callaghan
- British Army
- Callahan (disambiguation)
- Callihan
- Cellach (disambiguation)
- Cellachán Caisil
- Chief of the Name
- Cian O'Callaghan
- Con O'Callaghan (decathlete)
- County Meath
- County Monaghan
- County Westmeath
- Cromwellian Plantation
- David O'Callaghan (Kerry Gaelic footballer)
- David O'Callaghan (dual player)
- Denis O'Callaghan
- Donncha O'Callaghan
- East Clare
- Edmund Bailey O'Callaghan
- Eóganacht Chaisil
- Eóganachta
- Fivemiletown
- Gareth O'Callaghan
- Gary O'Callaghan
- George O'Callaghan
- George O'Callaghan, 2nd Viscount Lismore
- George O'Callaghan-Westropp
- Geraldine O'Callaghan
- Irish nobility
- Irish royal families
- Jim O'Callaghan
- John O'Callaghan (Medal of Honor)
- John O'Callaghan (musician)
- Joseph O'Callaghan
- Joseph T. O'Callahan
- José O'Callaghan Martínez
- Kells, County Meath
- Kevin O'Callaghan
- Kilian O'Callaghan
- Kinalea
- Kings of Munster
- Leland James O'Callaghan
- Lough Neagh
- Mallow, County Cork
- McGillycuddy of the Reeks
- Miriam O'Callaghan (media personality)
- Mollie O'Callaghan
- Munster Blackwater
- Murder of Sian O'Callaghan
- O'Callaghans Mills
- O'Donoghue
- O'Donovan
- O'Grady of Kilballyowen
- Pat O'Callaghan
- R. E. O'Callaghan
- Robert O'Callaghan (British Army officer)
- Sean O'Callaghan
- Sept (social)
- South Cork (UK Parliament constituency)
- Therése O'Callaghan
- Trim, County Meath
- Ó Ceallacháin