Oxygen is a dissolved gas with inverse solubility (less solubility with increasing temperature).  Since oxygen is a very corrosive gas it must be removed in the feedwater prior to the boiler.  Removal can be accomplished by increasing/controlling temperature and venting oxygen to atmosphere and by chemically scavenging the oxygen (using a reducing chemical agent such as sulfite).  Ideally, oxygen removal is primarily achieved by heating and venting - this reduces the risk of oxygen corrosion and the amount of chemical scavenger feed required.  Increased chemical scavenger is a concern for both cost and performance reasons (see TDS entitled Boiler System Oxygen Control).  If the feedwater tank has no provision for temperature increase/control, and operates at a temperature of <180 degrees F, it is a candidate to be retrofitted with a temperature increase/control setup.  A steam sparge tube or steam injector can be used to increase temperature and has proven successful in reducing feedwater oxygen levels and chemical use.