It can be very hard... not poly hard, but it will offer decent protection.
The oil hardens through a process of polymerization. Raw tung oil takes a while to cure and fully harden, but it will leave a fairly durable surface on most hard woods. You can also buy polymerized tung oil (sometimes called "tung oil finish") which has been boiled to speed up the polymerization process. It has mineral spirits in it to speed the drying time, but it will dry faster and cure harder than straight tung oil, with the slight trade-off of lending a slightly yellow tint. (Straight tung oil won't tint the wood.)
One big advantage of oil finishes: If the finish gets scuffed, scratched or worn, you just apply more finish, and it bonds with the old finish. Maintaining the finish is fairly easy.
Follow the directions on the can for how to apply it, as various brands will have slightly different instructions depending on what additives are in it. But essentially, you apply a thick layer of oil, allow it to penetrate, then wipe it off and allow it to dry. It can also be applied "French polish" style, by hand-rubbing many very thin layers.
Straight tung oil is nice to work with, because there are no solvents to stink the place up, so you can work with it indoors. Polymerized tung oils have solvents, so you're probably not going to want to use them in the house, unless you have a very well-ventilated room.
Caveats? Softer woods such as basswood will soak up a LOT of oil and it won't give them much protection. It's best used on harder woods. Closed-pore woods like maple and walnut take oil finishes beautifully, with no grain filling really necessary. Open-pored woods like mahogany, you're probably going to want to fill first. It's also not a completely maintenance-free finish, but as i mentioned before, touch-ups and repairs are fairly easy.
