The initial idioms of the letter O

on the horn	on the horns of a dilemma
on the horse	on the hot seat
on the house	on the hush
on the hush-hush	on the in, be
on the increase	on the inside
on the Internet, nobody knows you're a dog	on the jar
on the job	on the juice
on the junk	on the lam
on the leading edge	on the level
on the line	on the lines of (something)
on the lookout	on the loose
on the make	put on the map
on the march	on the margins of
on the mark	on the market
on the mend	on the merits