Atomic delegation, an object-oriented linguistic mechanism that allows the creation of dynamically defined classes of atomic actions is presented. When a type is modified, atomic delegation updates functionalities of the the types that delegate to the modified type. This mechanism permits dynamic binding and code reuse in atomic actions. The Sina language is used to illustrate the utility of atomic delegation in the modeling of a real-world problem, involving an office with a number of departments, using object-oriented techniques. Implementation issues are discussed.