For oil, Bo is reservoir barrels per stock-tank barrel. It is greater than one for a live oil because dissolved gas swells the oil and because of temperature. As pressure falls below the bubble point, gas leaves solution and Bo declines.
For gas, Bg is the reservoir volume of one standard volume of gas, in reservoir cubic feet per standard cubic foot or reservoir barrels per thousand standard cubic feet. It is much less than one, because gas compresses. Dividing a reservoir volume by the formation volume factor puts it in surface units. Both factors come from a PVT study, not from a rule of thumb.