is a freeware rhythm game originally created and self-published by Australian developer Dean Herbert.

Ouendan series of rhythm games, primarily involves clicking notes, which appear as circles, using the mouse cursor.

are created and uploaded by users, increasing the range and volume of the song library, which is a factor contributing to the game's popularity.

[6][7] In-game settings, called mods, can change gameplay in different ways—for example, by speeding up the song or decreasing the size of circles.

[6] Scores are primarily measured using their performance points (abbreviated as PP) , which account for various aspects of a player's skill.

[9][12] Users can add others as friends and chat in-game,[8] and the multiplayer mode allows groups of people to play beatmaps synchronously with each other.

[8][11] In osu!mania, a mode based on rhythm game series such as Beatmania[5] and Guitar Hero,[8] the player must press the correct keys on the keyboard when notes reach the bottom of the screen.

was created by Dean Herbert, an Australian developer known online as "peppy" or "ppy",[8] as a computer version of Osu!

[15] Herbert had experience creating video games prior to osu!, having made some during high school and university,[16]: 05:34  but has said he did not think it would become his job.

YouTube channel, the user interface of the game client and official website were undergoing full "revamps".

has continued to gain popularity since its release, being widely shared on video and live streaming platforms like YouTube and TikTok.

[15][20] An artificial intelligence VTuber called Neuro-sama became famous for playing the game, beating the world's best Osu!

has been noted for encouraging the use of unique input devices—although the game can be played with a computer mouse, it is often recommended to use a graphics tablet with a pen, which more closely emulates the gameplay of Ouendan.

favorably with 18/20 points in 2015, praising its simple gameplay and its expansive community, but complaining about a high skill floor and its unoriginal gamemodes, as well as the prevalence of Japanese music, which they felt was excessive.