A method of preparing an organic light-emitting device includes forming an organic emission unit on a substrate, and forming a thin film encapsulation layer that contacts an environmental element, and that includes at least one inorganic layer including a low temperature viscosity transition (LVT) inorganic material, wherein the inorganic layer is formed using a process including forming a pre-inorganic layer including the LVT inorganic material by providing the LVT inorganic material onto the organic emission unit on which the environmental element is located, performing a first healing process on the pre-inorganic layer at a temperature greater than a viscosity transition temperature of the LVT inorganic material, and performing a second healing process on the pre-inorganic layer having undergone the first healing process to increase a binding force between the environmental element and the LVT inorganic material, and to increase a binding force among the LVT inorganic material.