Early in his career as an independent researcher, Oesper continued investigations in analytical chemistry. His scientific accomplishments at this stage of his career included demonstrating the utility of the compound ferrous ethylene diamine sulfate in certain applications especially redox titrations. This compound replaced Mohrs salt (ferrous ammonium sulfate) in many applications because of its greater stability compared to Mohrs salt. He published these findings in 1947. Ferrous ethylene diamine sulfate is often referred to as "Oespers Salt", in recognition of Oespers scientific contributions.
A further contribution to analytical chemistry was his demonstration of the use of naphthidine as an indicator for certain chromate titrations such as for chloride ion. This improved indicator resulted in better testing results. He published this finding in 1934.