The initial idioms of the letter O

open-and-shut case	open (one's) mouth
open-ended	opener
opening gambit	the opening of an envelope
open the door for	opening
opera	operate against
operate from	operate on
operate	the operative word
operator	opinion
OPP	opportunity
opportunity knocks	opportunity knocks at every man's door
Opportunity knocks but once	Opportunity makes a thief
oppose	opposed to
opposite number, one's	opposite sex
opposite	opposites attract
in opposition	opt for