Is it recommended to apply oil paints directly to raw canvas?

You can, but it isn't always a good choice. Oil paintings are usually textural, most artists who work in oils build up layers and brush strokes are an integral part of the finished piece.

Depending on the amount of texture and the ground (canvas-paper, canvas-board, stretched canvas, or some solid surface, like hardboard) you can sand the painting slightly -if the ground is rigid, then add a coat or two of gesso. In the case of flexible grounds just use gesso, to smooth out the surface and allow to dry.

Once the gesso is dry you can paint a new painting over the old one with no worries at all.