In order to deal with business evolutions, it is important today to enable fast developments of new functionalities on the basis of existing Services reuse. In the context of information search within a federation of independent systems, we address the problem of automated discovery and orchestration of Services. We propose an approach based on a domain ontology, and on the Services semantics capture by the mean of ontology properties and algebraic expressions. We present an algorithm that generates orchestration plans, with characteristics of optimality regarding QoS. The approach has been validated by a prototype and an evaluation in the case of an Health Information System.