Oldcodex (stylized as OLDCODEX) was a Japanese rock band from Tokyo, Japan, formed in 2009. They performed the opening themes for the anime series Free!, Servamp and God Eater and the ending themes for Kuroko's Basketball and Togainu no Chi. Their band name indicates their wish to "create their distinctiveness" in rock music. They also take on a unique concept, having a painter draw on canvas while the main vocalist sing on live stages.

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Oldcodex was founded on October 21, 2009, by members Ta_2 (Tatsuhisa Suzuki) and R.O.N, who used to work with Ta_2 on his past solo projects. They debuted with the first mini album "OLDCODEX". They were accompanied by other members from another one named "ROSARYHILL", including R.O.N himself, YoHske and sae, though the last two did not stay from February 15, 2010, onward. On October 3, they put out their first full-length album, "hidemind", with 11 tracks in total. Many of their album covers were drawn by artist YORKE., who then became an official member of the band on November 1. Later on, the band witnessed drastic changes in the lineup, with new arrivals of 3 support members and R.O.N's departure on October 15, 2012. After 12 years of activities, the discography of Oldcodex includes 6 studio albums, a remix album, 3 mini albums, 18 singles and a digital single.

Oldcodex performed at Nippon Budokan on February 11, 2015.^[2]

On August 4, 2021, Ta_2 announced he was going on hiatus, with Oldcodex subsequently pausing their activities as well. This announcement came shortly after Shūkan Bunshun reported that he was involved in an extramarital affair with a female work associate.^[3] On August 30, 2021, Ta_2 released a statement apologizing for his "careless behavior."^[4] On December 27, 2021, it was announced that Oldcodex was set to disband following the release of the theme song for the upcoming Free! movie on the original soundtrack album which is scheduled for April 2022.^[5] Ta_2 and Yorke both shared their feelings about the announcement on the exclusive fanclub site, which was closed down at 12PM local time on March 1, 2022. They officially disbanded on May 31, 2022. The official website and Twitter account for the band were shut down on August 31, 2022. The band's website now redirects to the Lantis website.^[6]