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You don't have one. This is where the Barometric draft control would be located on your Boiler.

As a result of not having a Barometric Draft Control, the air fuel mixture will constantly change as the pressure inside the fire chamber changes. Since the chimney draft changes constantly with weather, barometric pressure and wind conditions at the top of the chimney, the amount of draft (or vacuum in the chimney) will change constantly. Since the top of that chimney is connected by a piece of connector pipe (Smoke Pipe) to the back of the heater, the pressure will change there constantly. Since the oil burner is connected to the front of the boiler, the pressure that is in the fire box chamber is constantly changing. and since the oil burner fan is pushing air for combustion into the fire box that is constantly changing the amount of air the fan delivers to the fire. When the fire box pressure is lower the oil burner fan will push more air into the flame. When the firebox air pressure is higher, then the burner fan will deliver less air to the flame. And this pressure is changing second by second, so there is nothing you can do to constantly adjust it. The barometric damper will do that for you.

Oil odors can change as the flame becomes rich or lean based on the amount of excess air that is delivered to the oil flame. So sometimes the odor will be stronger than other times. As a result of the age of the chimney and the possibility of the mortar joints leaking combustion byproducts thru those leaks in the mortar joints, I would strongly recommend that you get a chimney liner. That will improve the draft of the chimney in many cases. (by improve I mean make more draft). But that improvement will actually increase your oil usage because the more powerful chimney will suck more heat out of your heater. By adding the Barometric Draft control The cooler basement air will be going up the chimney while less air from the heater will go up the chimney. The draft control damper will open to let less heater air and more basement air go up the chimney. The damper, when properly adjusted, will constantly be moving open and closed as the chimney draft changes so the air moving thru the heater will stay constant. That way the fire box pressure will be at a constant pressure regardless of what is happening at the top of the chimney. You want that pressure to be about -0.01"wc over the fire and about -0.025"wc at the breach where the connector is located (smoke pipe). Your oil berher service person will need to make those adjustments for you.

This is as simple as I know how to tell that story. It may be a little complicated but the bottom line is that you want a Barometric Draft Control and a chimney liner in order to get rid if the odor.