When should I use gesso for my oil painting paper? What

>>3122924
You should use it if the paper doesn't have any, or if you want to create textures with it to paint over.

Gesso is a primer, that serves two purposes: a base for the paint to stick to, so your painting doesn't chip, flake or crack off the board/paper/canvas, and to provide a white background to work on.

>>3122924
Most pre-primed surfaces could use a couple coats of gesso on top tbqh. On paper especially (even expensive Arches Oil paper) the first layer of painting tends to be really uncomfortable, gesso helps with that.