oh, owe

Oh or oh is an expression of surprise, also used to attract attention when speaking to someone--
Oh! I forgot to check the time.
Oh, John, would you mail this letter when you go out?

Owe or owe ( rhymes with oh ) means to be in debt or indebted to--
 How much do we still owe on the car?
"You owe me three farthings, say the bells of Saint Martin's."

Now that you know that, you can say--
"Oh, we still owe the lawn service company."

Note--"ow" ( rhymes with cow ) is an expression of pain--
Ow! I stubbed my toe!

Note # 2--"O" ( rhymes with oh ) is often used in poetry or music to add weight or sincerity to an appeal--
"O, come let us adore him..."