John in NM
Use ferric acetate. Easy to make, pour vinegar in a jar and drop in a steel wool biscuit. Maybe rinse the steel wool with acetone or alcohol first to remove any rust inhibiting oil.
You'll end up with a odd smelling solution that reacts with the tannin in the wood to produce a dark brown color. Might have a greenish look, if so you can go over it with lye for a brown color. On oak I think it will be so dark you won't see a greenish cast, that is common on maple.
The only trouble is that this method is variable. Depends on how you make it, when you use it, and how much tannin is in the wood.