The IoT is a domain in exponential growth: both the number of connected devices and the quantity of data they produce are increasing. The heterogeneity of technologies involved, and the diversity of domains impacted raise interoperability concerns. The semantic web principles and technologies help tackling these interoperability issues, and ontologies like SSN have been used in several IoT projects. However, many existing IoT ontologies fail to comply with the good practices of the semantic web. After detailing such good practices, this paper proposes IoT-O, a modular core-domain IoT ontology. IoT-O is then showcased in a home automation use case: it is used to semantically describe the devices of the system, and to guide the decisions of an autonomic agent.