When I start an oil painting by sketching it digitally, I go through a process of transferring the digital image to the traditional surface. There are a few ways to accomplish this task; Some artists use a large format printer and mount the prints to a work surface, some people use a projector or light box, and some people (including a younger version of me) use the laborious grid method. There really isn't a wrong way to do it, but I prefer the very easy graphite transfer method, which I'll explain.
A quick note about my tools: For an oil work surface I most commonly I use Strathmore 500 Series illustration board. It's got a great smooth surface like masonite or sanded wood, but is easier to find at my local art store on short notice and lighter weight for easier framing and transport. The other items I use for this process are my printer, a 7B pencil, an HB pencil, and some acrylic matte medium.
Once I have the finished digital drawing the way I like it, I print it out; when my printer is smaller than my work surface, I print the drawing out in several pieces and reassemble it with clear tape. I work these drawings up at high resolution (450 dpi) at their actual size, so I have lots of detail in the prints.
Once the drawing is printed out and reassembled, I take a soft 7B pencil and make a light coating of graphite all over the back of the printed drawing. when I trace over the front side with a harder HB pencil, the graphite will be lightly transferred onto my work surface. If you're a little less cheap than I am, you can also use transfer paper which is sold in rolls or pads.
Typically the light traced lines by themselves feel a bit stiff or aren't dark enough, so I'll use my HB pencil to draw over the transferred image in areas I think need more emphasis or definition.
Lastly, since the illustration board by itself would absorb the oil in the paint and eventually degrade, I seal the surface with 3-4 coats of acrylic matte medium after the drawing is transferred. This helps keep the illustration board intact and seals the graphite drawing.