Koji Kikkawa, who plays Kamen Rider Skull/Sokichi Narumi in the upcoming film, Kamen Rider x Kamen Rider OOO & W feat. Skull Movie Wars Core, along with Maki Ohguro, will sing the theme song of the upcoming film! As of now, there is no title for the said song, but it is confirmed that it is said to be released this December 15.
UPDATE: The lyrics of the said single is said to be written by Ms. Ohguro while the song will be composed by Mr. Kikkawa. Its been Maki's dream to become an artist like Koji Kikkawa (Maki debuted in 1989, while Koji debuted in 1984 with his single, Monica).
"I am literally dancing from joy to be able to sing with Koji Kikkawa." she said.
Koji Kikkawa said that he's liked Maki's songs before this was decided and feels like he's able to study a womans heart through this collaboration. They both seem to have a mutual trust and respect for each other.
Source: alternate-zer0, Animation Blog OCN via DecaEnd