Absolute differential excitation cross sections of atomic oxygen (3P--1S transition) by electron impact have been measured by a crossed-beam method. A thermal dissociation method was used for generating atomic oxygen. The energy and angular range measured were from 10 to 30 eV and from 30¿ to 150¿, respectively. A several theoretical results were compared with the present measurements. It is found that the present results do not agree with the theoretical predictions in angular distribution as well as magnitude of total cross sections. The theoretical values are generally smaller than the measurements by a factor of two.