I have some pics in my profile of some cages I have had over the years. The pics labeled "Current Tanks" and in no way current and only has cypress mulch in it. the pics labeled "Aru GTP" has a the mixture I am about to describe but I started adding more cypress to the mix than what is in that pic. I need to update my pics, those are old.
I like using a mixture of cypress mulch, eco earth, and repti bark. I mix each in a big tub adding the right amount of each until I get the desired look. Mixing them makes them appear much more natural. It also helps eliminate issues like the dustiness the eco earth can cause when used alone. And will hold moisture like a champ. It may not be the cheapest method but I like its appearance.
Oak is fine to use.