The trend towards deregulated electricity market in both Korea and Japan is first introduced, and then the technical developments in operation and maintenance (OandM) of nuclear power plants (NPPs) are reviewed from the consistent pursuit of improving safety and economy to meet with this change of business environment. The overview covers the present state of improving OandM technologies for light water NPPs, the on-going issues of the countermeasure for aged plants and restructuring of plant maintenance practice. Lastly, some emerging researches are also introduced for future plant technologies as well as for dealing with social aspects.