This depends if the oak door is interior or exterior. Oak is usually used more for interior doors as it can be prone to cracking and splitting when used for exterior joinery. basic options available for oak interior doors are lacquer / varnish or oil / hardwax oil. The right finish for your door depends on personal taste regarding the look and ensuring that the finish is suitable for the expected use of the door.
For interior oak doors, solid wood and veenered, most people go for a door oil such as
Osmo Door Oil
or
Manns Premier Door Oil
. These products seal and protect doors from dirt, grease, scuffs, moisture, hand and finger marks. They are very easy to apply and maintain. These oils tend to darken the appearance of the oak giving it an almost damp like appearance and will enhance the natural character and grain of the wood. To better retain the natural, untreated look of oak doors, consider
Osmo Door Oil (Raw 3033)
, this has been specifically designed to offer the same level of protection as the standard Osmo Door Oil whilst retaining the untreated look.
Interior door varnishes or lacquers can offer better protection in heavy use or commercial environments. Unlike door oils however, if the varnish or lacquer becomes scratched, chipped or worn, there is little that can be done to repair the damage whereas oils can simply be cleaned and re-oiled to produce an invisible repair to scratched or worn areas.
When applying a door oil,
always
do a test area to assess product suitability and finish before starting the project.