Oco literally means "hollow". It's a fun and unusual word that you won't find in every song, and it has a great sound that plays with the title, "Un Poco Loco".
In the context of the movie Coco, this word has a clever double meaning. While it could refer to the characters being literal skeletons, the line "Não sou tão oco assim" ("I'm not that hollow") is used to say that there's more to someone than meets the eye—that they have feelings and substance, not just an empty space inside.
This song is a super fun, slightly chaotic duet about someone driving the singer completely crazy, but in a playful way. It's all about a back-and-forth where one person says absurd things, and the other just has to go along with it.
Imagine asking someone, "Que cor é que o céu tem?" [What color is the sky?], and they insist, "Vermelho dizes que é" [You say it's red]. Or when you ask where to put your shoes, they tell you, "'na cabeça', olé!" ['on your head', olé!]. It's no wonder the singer feels "un poco loco" [a little crazy]! The whole song builds on this silly energy, with the singer admitting, "Digo sim a tudo, vê-se" [I say yes to everything, you can see], because the other person's logic is just impossible to argue with. It's a celebration of being a little bit silly and embracing the madness of love and friendship.