Linseed oil or tung oil is what your looking for. I prefer tung oil and carry the brand "waterlox". The oil is absorbed into the wood and keeps it from supple and from drying out while offering a water shed and uv protection. the first coat will soak in like a sponge, the 2nd coat not as much, and then if you would like a furniture finish that really looks good, sand the cabinet down with some 80 to 100 grit and rag apply additional oil to finish the surface.
This will keep your wood cabinet looking like a fine piece of furniture for the lifetine of the spa.