I think Simon has it right: the error is in the fuel type and the weight difference assumed by FSX.
I also came across this: http://www.ov-10bronco.net/faq.cfm
Under the weapons question (oddly enough), I found this: "The centerline station (only) can carry one 150 US gallon or one 230 US gallon drop tank. The under wing hardpoints can each carry a 100 US gallon drop tank. The five self sealing bladder wing tanks totalled 252 US gallons."
And also this: "The cargo capacity of an OV-10A is 3,200 pounds and 110 cubic feet." Interesting.