What is your advice on teaching the sight word "on"?

I am a student teacher and I have been working with one girl who can read the word "on". But when I asked her to spell it at a different center, when we were writing sentences, she had absolutely no clue how to spell it and sounding it out did not help her at all. I'm thinking that she doesn't get far enough to the letter "o" on the letter practice software. I think a book that focuses on the letter "o" might be helpful to learn the sounds of "o"

Do you have any advice for me on how to teach her the word on?

We have already been working on using the word on in a sentence for the past week. She knows what the word is when its in front of her. She just had no clue how to spell it when I asked her to twice at different times today.