Off color
Feel sick

Off the hook
Stay away from problems or impacts of what they have done.

Off the shelf
The products that can be used with no set up required.

On a charge of
On an accusation of

On a large scale
Big amount / size / level

On all fours
Crawl

On all sides
From every direction

On edge
Serious or worry with the coming situation

On ice
Stop or keep it for the future

On the ball
Understand the situation very well

On the blink
Mulfunction, out of order (use with machine)

On the nose
Just in time, Exactly right time

On the hook
Responsible for any impacts of what they have done.

On thin ice
On a risky situation.

Once in a blue moon
Something happen very rarely

Out of health
Not healthy

Out of the blue
Don't feel sad anymore

Once and for all
Hope this time will be the last / hope to have the conclusionl

On the second thought
Change the mind about something

On the tip of your tongue
Try to remember something but not quite remember it

On tiptoe
Walk quietly

On your last leg
Dying, Last moment of life.

Off the hook
Out of difficult situation

One bird in the hand is woth two in the bush
Better to keep small thing you have than to leave it for something            better

One of a kind
Special / Only one among all

Out of pocket
Out of time / Out of contacted

Out of the box
Varied from the normal. Usually used with thinking