It all depends on what you want the oil to do. When you put a finish on a traditional rifle, one of the methods of treating the wood is with different oils. There is Tongue Oil, Linseed Oil, and other combinations. For instances when I did my Renegade stock, I rubbed coat after coat of boiled linseed oil into the stock until the color and finished was obtained.
When you have a treated stock with a sealed finish, there is really no reason to oil the stock as the varnish, wax, polyurethane, or what ever sealer you have will not allow the oil to soak into the wood anyway. It might dull the finish also. So be careful. If you need to know, check in an area of the stock that will not be noticed.
Some finishes might be harmed, but I have put oil on my rifles and never noticed a problem yet. I even take Birchwood Casey Sheath and will sometimes wipe down the stock of my oil finished rifles. Since it is an oil finished, it does not hurt anything.