As long as the oil is well and truly cured, you can do it. You'll have to scuff the entire surface with fine (320 grit) paper to get good adhesion, and make doubly sure you wash any trace of wax off first with solvents. To guarantee adhesion, a wash coat of shellac applied next will help. Not Bullseye Shellac off the shelf of the big box store, either. Get blonde shellac flakes and dissolve in alcohol, only as much as you'll need. No guarantee that the stuff in the can is only a day or two fresh!
If the gun has been subjected to "protection" with WD-40 or (shudder) Armor-All, forget it, even if only applied to the metal. Just wax the heck out of the stock and use it. The silicone in those products is insidious stuff and even sanding it down to bare wood won't dismiss all of it. You'll get fisheyes in your varnish finish.
As far as varnishes go, get the very best you can find. Around here, where boats and varnishing abound, Epifanes is the all-time favorite. Very high in solids content and remarkable UV protection. Also rather expensive- $30-40/qt., but it's the best IMO. How much money do you want to save when the protection of your pride-and-joy stock is the issue?
Spar varnish is better by far than urethanes. It's more pliant and as a result will not craze if (when) the stock moves from radical shifts in environmental wetness/temperatures.
I have finished stocks with spar varnish that uninformed folks thought were done with a "hand rubbed oil finish." It, like most things in life worth doing, requires more than a lick-and-a-promise-then-reach-for-the-Tru Oil-approach. The end results are worth it though.
These are my opinions, and to some may not be worth a bucket of warm spit! It's all good.
