This will sound crazy but it works incredibly well.
Couple ways to start here.
1) wrap it in a towel/rag whatever and a black plastic garbage bag. Put in the hottest sun you have.. even dash of your car. Let it bake a day.
Disassemble and clean metal with mineral spirits or acetone. Bore too.
Stocks can be funny. You have a shooter or collectible? It matters here.
Assuming a shooter, you can do well with mineral spirits but oil will bleed for years.
A bit more aggressive is acetone and denatured alcohol mixed 50/50 wiped on wet and then off.
Dont like chemicals? No problem. Use Smooth Fast Orange hand soap. This is the shit! Slather on, work with a soft nylon brush, wipe off. Repeat as necessary. I've even boiling water rinsed after and towel dried to no ill effect.
Real bad oil stains I used old fashioned whitening powder (powdered gypsum) and 50/50 acetone, denatured alchohol. Make a wet paste, paint it on good, bag it up and let it dry. It simply sucks out the oil. You can use baby powder too..
When done I'd simply reoil with whatever was original and rub with 0000 steel wool.
Worked pretty well on a few dozen I've done over the years.
Attached File Attached File Attached FileResults of the above process. Note you can now see the cartouches.