On top of (something)
In addition to something, along with something.On top of everything else my boss wants me to work on Sunday.
On top
In the lead.The boy was on top of his class when he was in university.
On tiptoe
Standing or walking on the front part of the feet.I walked around the house on tiptoe so that I would not wake up my family.
On time
At the scheduled time, exactly at the correct time.Our train arrived exactly on time.
On thin ice
In a risky situation.The woman was on thin ice with her company after she continued to come to work late every morning.
On the whole
In general.On the whole, I think that the plan is a good idea but I would like to study it further.
On the way (somewhere)
On the route to somewhere.We got something to eat on the way to the airport.
On the warpath
Very angry, looking for trouble.Our boss is on the warpath today so you should stay away from him.
On the wagon
Not drinking alcohol (usually used for someone who has a drinking problem).The man has been on the wagon for seven months now.
On the verge of (doing something)
Just about to do something.I was on the verge of quitting my job when I suddenly changed my mind.