Good question. Builders are too cheap to front the cost of it and the oil companies (and this includes the small local guy) have a conflict of interests. Not selling you an OAK allows them to sell you more oil. They should also install an oil vent damper on the flue, but that costs up front and saves oil also. An OAK (they refer to it as a fresh air kit) for my Beckett burner retails for about $150 additional tube and vent cap are about another $50 to $75. The local oil dealer wants $900 for it, installed. Installation is a piece of cake, maybe 45 minutes of work. Oil vent dampers can also be had for about $150, retail. Installation involves some wiring.