AAO is Assistant Audit Officer in CAG offices, while ASO is Assistant Section Officer in ministries and departments.
AAO is a gazetted Group B post with a higher pay level; ASO is non-gazetted Group B.
AAO work involves auditing government accounts, while ASO work involves noting, drafting and coordination in the secretariat.
AAO needs a desirable qualification in some streams, while ASO is open to any graduate.
Both are among the most preferred posts in SSC CGL.