Regret nothing ~Tighten Up~ is an ending theme (ED) from Kamen Rider OOO. It was performed by Shu Watanabe. Regret nothing ~Tighten Up~ (Regret nothing ~Tighten Up~, Riguretto Nasshingu ~Taitan Appu~) is the theme song of Kamen Rider OOO Tatoba Combo and one of the ending themes of Kamen Rider OOO. This song was recorded throughout late September 2010 and recording completed on October 2, 2010. Shuhei Naruse is the song's producer and it is sung by Shu Watanabe under the name of his character Eiji Hino.