O. N., or omn. noct., omni nocte, every night.
Omn. quadr. hor., omni quadrante horæ, every quarter of an hour.
O. O. O., oleum olivæ optimum, best olive oil.
Ov., ovum, an egg.
Oz., the ounce avoirdupois.
P. æ., part. æqual., partes æquales, equal parts.
P. d., per deliquium, by deliquescence.
Past., pastillus, a pastil, or ball of paste.
P., pondere, by weight.
Ph. D., Pharmacopœia Dubliniensis.