What ever floats your boat or what you have to hand, (talking as a cabinet-maker for the last 30 years to the rich and greedy) oil is fine but as they say, it soaks in and you re-do it when you have to, wax is about the same team but smells a little better, but you have to be sharp to not build it up in the corners or it looks a tad ***** and stuff sticks to it. Varnish is easy to get hold of and several thin coats rubbed back will give a deeper finish than all the above, Or you take it all out and get it lacqured at a workshop with some 2 pack lacqure, that way you can go for whatever sheen level you want, I would go for 10 %
Whichever one you go for, top tips are, if you go for oil it will be one arse of a job to change your mind as it will of soaked in, the same with wax... always do the back or inside of something first, if you don't like it you've not spoiled a good one... and, if you do one face of something you should do the reverse as it will only be sealed on one side and still take in and loose moisture from the un-treated side so will bow. Plywood is dead and done all the moving it will ever do so has no need of a good feeding of anything, it will take in moisture though, so will rot, to this end it still needs treating.
I whitter, hope that helps.....