One day

One day is an insert song featured in the Black★Rock Shooter OVA. The song is performed by Nagi Yanagi (then known as nagi). The original lyrics, composition, and arrangement were done by ryo, with the English lyrics done by Taishi Fukuyama.

The song is included on both releases of Black★Rock Shooter Animation; Original Soundtrack & Remixes, along with a remixed version by Shinichi Osawa.