The absorbancy of paper is dependent on its weight.
For painting purposes, I suggest to use paper with a minimum weight of around 300 gsm (or g/m2) and up.
The ratio of water to gesso is more a personal preference, since you will want to tape the paper to a flat surface.
The proportion of water determines the coverage and texture. Since you want to use oil paints, this ground should not be too thin (usually, I don't thin the gesso down at all, but this is naturally dependent on the brand and type), but you can apply the gesso in multiple thinner layers, or one thicker one, if, for example, you would like to imitate a canvas-like structure (applying the gesso in a thick vertical and horizontal layer).
As a rule of thumb, apply as much gesso as is needed to keep the paper from shining through: it's a ground layer, the oil paint should not be able to come into contact with the paper directly, as this will eventually cause problems of both aesthetic and conservatory nature (as the oil will be absorbed by the paper)!