Similar to a.l., a.s.

om - omne mane - every morning
on - omne nocte - every night
o_2 - both eyes, sometimes written as o2
p.c. - post cibum - after meals
p.m. - post meridiem - evening or afternoon
p.o. - per os - by mouth or orally
p.r. - per rectum - by rectum

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