Why does 오다 turn into 와요?

Sorry if it’s a stupid question, I’ve picked up the Talk To Me In Korean Real Life Conversations E-Book as previously advised in my first question on here. There could just be several grammatical rules that I simply haven’t learned about yet.

I understand that :

아요 – If the verb stem’s last vowel is ㅏ or ㅗ you add 아요.

어요 – If the verb stem’s last vowel not ㅏ or ㅗ you add 어요.

여요 – Add this to the verb stem that ends with 하.

But why does the 오 suddenly get a whole new vowel added to it?

Thanks to anyone who spends the time to help me on this 🙂