Did you finish all sides including the ends? The most common cause of failure is moisture coming in from behind.

The best stuff to use is marine spar varnish in the latest formulation. Go to your local marine store and ask what to use on a wood boat deck, and then do that. It will be expensive but it will last. All finishes eventually crack but the marine can be repaired repeatedly by sanding and re-application.

A tonneau cover will protect it from the weather.

Also, park it on a slope when it rains so the water doesn't pool.

Oh, and oak is very porous. It need a penetrating finish to fill and seal the pores up or it wicks up moisture very fast.

Paint & Body Shop moderator
A lone amateur built the Ark. A large group of professionals built the Titanic.