On the air: broadcast, transmitted, or shown on TV or radio.

our favorite singer is __ right now. Switch on the TV!

on the fence: undecided, unsure, uncertain

Jack: Have you decided which university you're joining?
Chloe: No, I'm still __________.

on the warpath: very angry, ready to fight, ready to go to battle

The parents are __ since they heard about the big increase in school fees.

on the level: honest, truthful, straightforward

The interviewer was __ about why they could not hire me. It seems you must have a PhD to get that position.

on the ball: very good, competent, alert, knowledgeable

Wow, you're really __ today! That was the best speech I've ever heard you deliver.

on the sly: secretly, deceptively, without anyone knowing

Mary and Bill were seeing each other __________. Their spouses didn't know anything about it.

on the mend: recovering, healing, getting better

Jim: Hey, how's your grandfather now? Is he out of the hospital?
Bob: Yes, he's back home and __________. Thanks for asking.

on the fritz: not working, broken

I was really annoyed because my laptop was __ this morning. I couldn't finish my report.