SxOxU is a music project and the solo endeavor of Japanese musician Matsumoto Sō, who was born on December 22, 1978. Matsumoto is primarily known as the vocalist and guitarist for the rock band GOING UNDER GROUND, for which he writes and composes many of the group's songs. The SxOxU project was launched in the summer of 2009.

The project is most recognized for its connection to the long-running anime series Katekyo Hitman Reborn! SxOxU performed the seventh opening theme song for the anime, titled Funny Sunny Day. Specifically, the Japanese version of this song, listed as Funny Sunny Day (REBORN! Japanese Version), was used as the opening theme for episodes 154 through 177, which aired from October 2009 to March 2010.

Funny Sunny Day was released as a single on November 11, 2009, under the Pony Canyon label. The lyrics for the song were written by Mika Arata, with composition credited to SxOxU and arrangement by Hidaka Tooru of the band BEAT CRUSADERS alongside SxOxU. The single peaked at number 32 on the Oricon charts.

The project also released a self-titled EP or mini-album on December 2, 2009, which includes the track Funny Sunny Day. The full track listing for this release includes Don't Want To Be Alone, Roller Coaster, Summer's Gone, VOID, Departure, Remember, and an acoustic version of Don't Want To Be Alone as a bonus track.

Musically, SxOxU is described as a project that embraces a 1990s alternative guitar rock sound, characterized as chaotic and emotional band music. The album was produced by Hidaka Tooru, and the recording musicians included Hisako Tabuchi on guitar, Naomi Iwasaki on bass, Mizue and Hiroki Goto on drums, and Hiroshi Fujiwara on bass.