a bone (used chiefly in Latin names of individual bones, e.g. os trapezium)

an opening or entrance to a passage, especially one at either end of the cervix of the uterus

the fifteenth letter of the alphabet

nought or zero (in a sequence of numerals, especially when spoken)

two seven o seven point six

a shape like that of a capital O; a circle

he made an O with his mouth

the chemical element of atomic number 76, a hard, dense silvery-white metal of the transition series

the chemical element osmium

the method of calculating dates using the Julian calendar

(in calculating dates) Old Style

open-source

operating system

Ordinary Seaman

(in the UK) Ordnance Survey

(as a size of clothing) outsize

out of stock

overseas