が来るのを待ってる

waiting for [someone or something] to come

→ が
phrase Rare #22,280
waiting for [someone or something] to come The subject or thing being waited for comes before が. 待ってる is the casual contraction of 待っている, meaning “is waiting.”
JA: 友達が来るのを待ってる。
EN: I'm waiting for my friend to come.

トムレッカーが来るのを待ってるんだ

Tom is waiting for the tow truck to arrive.

レッカー車が来るのを待ってるんです

I'm waiting for the tow truck to arrive.

誰かが来るのを待ってる

Are you waiting for someone to come?

友達が来るのを待ってるんです

I'm waiting for my friend to arrive.

トムが来るのを待ってる

Are you waiting for Tom to come?