"One Fine Day" was written by Jung, who contributed to its composition along with Two Face; the latter arranged the song. The track is a breakup ballad where the narrator recalls his former girlfriend and recollects his memories of their relationship, which he describes as "fine days". The track infuses "string and orchestral elements" in the song previously not found in CNBLUE's music. It is composed in the key of B-flat major using common time with a tempo of 71 beats per minute.
