Mortality for oldest-old age (aged 80 or over) in Korea has never been studied mainly due to lack of data leaving its quality aside. The rapid aging recently occurring in Korea, which no other country has never experienced, must introduce a drastic change in the structure of future population pyramid, requring a careful investigation on mortality level, pattern, and trend for the oldest-old age in Korea. For mortality trend, we need to extend the past mortality data, which has provided only an open-ended age interval 80 or 85 years and over, to age-specific mortality up to 115 years old. A prerequisite for such an extension is constructing a Korean standard mortality by which one can figure out the mortality level and pattern of the oldest-old age. A Korean standard mortality is proposed by applying one relational model and eleven functions from which we select best models for each sex in terms of three measures of fits and three consistencies of mortality. Then we extend the mortality of the open-ended age intervals by providing a method to avoid the longitudinal consistency of mortality.