"Only U" was composed by Leeteuk and Lee Jae-myoung (who previously wrote for the group's tracks "Marry U" and "Happy Together") with the lyrics by Leeteuk and Donghae.
"Only U" disusun oleh Leeteuk dan Lee Jae-Myoung (yang sebelumnya menulis track kelompok Marry U dan Happy Together) dengan lirik oleh Leeteuk dan Donghae.