Oh, Fionna is a song that Prince Gumball (actually Ice Queen in disguise) sings to Fionna in "Fionna and Cake." It was sung flying through the air on Lord Monochromicorn, and was accompanied by Cake playing her dulcimer.
Prince Gumball: 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 will 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 and Fionna: I won't let anything in this world, keep us apart.