nounAinmfhocal1letterOothe letter Oan litir Othe 'o' is missingtá an t-'o' ar lárthere's a fada on the 'o'tá fada ar an 'o'2position in list, sequence, ranking etcOAppendix OAguisín O3chemistryceimicsymbol for oxygenO4shapeOan O-shaped framefráma O-chruthach, fráma ar chruth O5 (also nó oh)zeroa náidcmuneamhnímasc4cmu6medicineleigheasblood typeOblood group Ofuilghrúpa OO negativeO diúltachO positiveO deimhneachFor usage examples see ASubformsSainfhoirmeacha→ O'