Component-based software engineering is a paradigm that fosters software flexibility and emphasizes composability and reuse of software components. These are runtime units that provide services and, in turn, may require other services to operate. Assembling components consists in binding compo-nents' required services to provided ones to deliver composite services with added value. Building a composite service is a challenging task as it requires identifying components and services that are compatible, binding them to implement the service, and describe it for discovery. For that, the vocabulary used to describe component-based services (i.e., services offered by components or assemblies) must support the description of required services, and descriptions must be combinable in order to automatically generate composite service descriptions. However, existing solutions are limited to the description and composition of provided (and not required) services. In this paper, we consider ontologies to describe component-based services implemented by component assemblies. After comparing existing service ontologies, we present an extension of OWLS called Comp-O. Through a proof-of-concept, we demonstrate how the added semantics can be handled to automatically build composite service descriptions.