I believe "oi" 「おい」 can be translated as "hey," too, and is potentially used more commonly than the former.
Just a small note/suggestion. I've translated some manga, too, but I consider my work to be subpar at best as a noob.
Mostly commenting, 'cause I love the fact that you're using manga translation as practice, too. (I hope the next one I translate has furigana.) Glad to see I'm not alone in using this for practice! Awesome! x'D