| Oncostellae | A Coruña |
| Oncovision | Valencia |
| Ondata International, S.L. | Madrid |
| Ondho | Barcelona |
| ONE digital consulting | Madrid |
| One2One Digital Strategy, S.L. | |
| Onechain Immunotherapeutics | |
| OneCoWork | Barcelona |
| Onenpro S. Coop. | Gipuzkoa |
| Onext | |
| Oniria Therapeutics, S.L. | |
| Online Marketing A.C.D, S.L. | Madrid |
| OnRetrieval | Madrid |
| Ontech | Sevilla |
| Ontic Soluciones, S.L. | Albacete |
| Ontinet.com | Valencia |
| Ontrace Intelligent Video Analytics | |
| Ontruck AI Tech | |
| Onum | |
| Onurabat, S.L. | Biscay |
| Onversed Technologies, S.L. | Málaga |
| ONYX8 Digital Agency | |
| Opcionalia Soluciones Avanzadas | Granada |
| Open Canarias | Santa Cruz de Tenerife |
| Open Cloud Factory | Biscay |
| Open Document Management System, S.L. | Balearic Islands |
| Open Future | Madrid |
| Open Xarxes Coop. V. | Valencia |
| Open2b Software S.n.c. | Pontevedra |
| OpenBootcamp, S.L. | Valencia |
| Openbravo, S.L.U. | Navarre |
| OPENETICS International, S.A. | Barcelona |
| Openfinance | Valencia |
| OpenHost, S.L. | Ourense |
| OpenHR | Madrid |
| openTrends Solucions i Sistemes, S.L. | Barcelona |
| OpenVidu | Madrid |
| OPEX Informática | Madrid |
| Optare Solutions | Pontevedra |
| Optima Solutions | Madrid |
| OptimaSys Group Spain, S.L. | |
| Optimaweb | |
| Optimoclick | Barcelona |
| Optimus Price | Barcelona |
| Optimus, S.A. | Girona |
| OPTYMA - Proveedores de Presencia en Internet, S.L. | Madrid |
| Oracle Spain | Madrid |
| Orange España | Madrid |
| Orbit Consulting Group, S.L. | Madrid |
| Orbyn, S.L. | Madrid |