It's sometimes more difficult to mess with an existing finish than to refinish.. Anything you use to darken the stock will darken the oil thats already there. That will lessen the clarity of the finish and blott out the figure of the wood.. I would carefully remove the oil finish and using an oil (petroleum)based stain, I would darken the wood. The oil finish can then be reapplied and all will be well. That is if what you have is an 'on the wood' finish not an 'in the wood' finish'. If there is NO finish build up 'on top' of the wood then the finish is said to be 'in the wood' and a good quality solvent based stain for gunstocks should darken it just fine.. It would be wise to thoroughly clean the stock with acetone (use outside) before staining. I prefer 'Pilkington Clasic Spirit Stains' as sold by Brownells but have sucessfully used the 'R.Gale Lock Co' stains. These last stains have a 'dark walnut' color that would probably provide the level of stain you are looking for. Good luck from the gunnut69