You can't get osu on the ipod without jailbreaking it. When you jailbreak it, the only thing you get is an app. That app is like the app store except the apps on there are....different. You get osu through there.
Why not just jailbreak it? After jailbreaking mine, I don't understand how people could use their ipods without jailbreaking. I can change the my music by pressing both volume buttons. I download videos and music while I have internet access and watch/listen to the videos/music while offline. You have access to so many things after jailbreaking.