DAGO is an acronym which stands for Department of the Army General Officer. It is not typically used outside of the Department of the Army headquarters and staff sections, however it is sometimes seen in memorandums and other forms of correspondence to lower level echelons. It is also sometimes used to refer to General Officers that are part of the Department of the Army by personnel that are within other sections of the Department of Defense.
There are also certain orders issued by the Department of the Army that are called DAGOs or Department of the Army General Orders that pertain to base closures, unit citations, the creation of JROTC units, certain awards such as the Medal of Honor and Legion of Merit and structural changes to the Army. These are listed by year on the Army Publishing Directorate's website.