Circular dichroism (CD) spectroscopy in the soft x-ray energy region is a new tool to study the local structure of chiral materials. In this paper, we introduce a method to measure high-quality CD spectra in the oxygen K-edge energy region. Characteristic CD spectra of thin films of the amino acids L-tyrosine and L-aspartic acid are reported and compared with those of films of L-alanine and L-serine. The signals from the oxygen 1s → π∗ transitions of COO-, which is a common moiety in these amino acids, reflect the local geometry of each amino acid.