The Oak door is a door that can be made in the dungeon of a player-owned house with the Construction skill. Constructing these is recommended for training instead of oak larders once the level requirement is reached. When made with the Butler it is possible to make 360k experience per hour.
At current Grand Exchange prices, it costs 4,200 coins to build an Oak door. This is 7 coins/XP (without bonus experience). With the use of a butler and bonus experience, training with oak doors costs 3.76 coins/XP or 3.7 coins/XP after completing Love Story.