Oh, Fionna is sung by Prince Gumball to Fionna in the episode "Fionna and Cake," but this song was not sung by Gumball, it is the Ice Queen in disguise of Gumball, considering the romantic implications of the song. During this song, Cake's Dulcimer was used.
Prince Gumball (Ice Queen): I feel, like nothing was real, until I met you.
I feel like we connect, and I really get you.
If I said,"You're a beautiful girl," would it upset you?
Because the way you look tonight, silhouetted, I'll never forget it.
Oh, Oh, Fionna. Your fist has touched my heart.
Oh, Oh, Fionna.
I won't let anything in this world, keep us apart.
Prince Gumball (Ice Queen) and Fionna: I won't let anything in this world, keep us apart.