D'ocê is a charming, colloquial contraction of "de você" (of you/from you), commonly used in informal Brazilian Portuguese, especially in certain regions.
It adds a touch of intimacy and regional flavor to the lyrics, making the expression "Ai que saudade d'ocê" (Oh, how I miss you) feel more personal and heartfelt. It's a great example of how spoken language can differ from formal grammar in songs.
The whole song is one long excuse to send a message. The singer is away somewhere and tells his sweetheart not to be startled if a hummingbird turns up: "Não se admire se um dia / Um beija-flor invadir / A porta da tua casa" (Don't be surprised if one day a hummingbird comes through the door of your house). The bird is him, or at least the kiss is. "Fui eu que mandei o beijo" (I'm the one who sent that kiss), and then the line the title comes from, "Ai que saudade d'ocê" (Oh, how I miss you). D'ocê is a countryside contraction of você, and Mariana Nolasco leaves it exactly as Vital Farias wrote it.
Then he asks for an answer, and he is oddly specific about the logistics: write a letter, "Bote logo no correio" (put it straight in the mail), and copy the same words back to him. The third verse is where it stops being sweet. He remembers her crying every time he packed to leave, and himself crying on the road, and lands on something that isn't really a resolution: "Viajar é minha sina / Eu gosto mesmo é d'ocê" (Travelling is my fate, the one I really love is you). Two facts that don't fit together, sung over one guitar as though they might.