As Dave noted, white oak is rot resistant while red oak is not. As such, the best bet if you have picked out red oak is to seal the entire board, top, bottom, sides, and ends. Any drying oil (like tung oil, boiled linseed oil, etc.) is going to have to be reapplied every year, and gives little UV protection. A semi-transparent deck stain or an oil based deck stain will work better as it has UV blockers added. Something like Sikkens is regularly recommended by deck builders for longevity (3-5 years)on projects where you don't want it to look painted.
I have no idea about the long term effectiveness of motor oil, but I have an old workbench that beads right up if water is spilled on it because someone clearly tore down an engine on top of it and soaked it through with oil. Paraffin or Anchorseal (a wax emulsion paint) are other options.
