Antique slant top desk

It most assuredly is made of oak.  The drawer front is even quarter sawn oak - often called tiger oak.  That's a more valuable cut of wood.  It's prettier and it's also cut slantwise from the tree so takes more wood to get a piece like that.  It also doesn't warp if cut that way so that's an extra advantage.  Your piece was a steal at $40 since it has all the inside parts.  It's arts & crafts mission style and would be worth $800 or so here on the west coast.