| NuonSoft | Flemish Brabant |
| NVISO Security | |
| NYN IT Consulting | |
| Nyxoah S.A. | |
| Nyxoah S.A. | |
| Obelis Group | |
| Obulytix | |
| Océ Software Laboratories Namur S.A./N.V. | Namur |
| Ocean Impact Organisation | |
| OCTIMET Oncology N.V. | |
| Odisee | |
| Odit Systems | Antwerp |
| Odoo S.A. | Walloon Brabant |
| Offimac ICT Solutions | |
| OHMX.bio | |
| OMNINET Technologies N.V./S.A. | |
| OMP | Antwerp |
| Onco3R Therapeutics | |
| OncoDNA | |
| Oncomfort | |
| One Assessment | |
| One2Web | East Flanders |
| OneBonsai | |
| oneM2M | |
| OneTec group | Brussels-Capital |
| Online | Flemish Brabant |
| Ontoforce | |
| Oonex S.A. | |
| Opal Solutions S.R.L. | |
| Open-Future B.V.B.A. | Flemish Brabant |
| Openforum Europe | Brussels-Capital |
| OpenGraphy | |
| OpenIT | Namur |
| OPNS S.A./N.V. | Brussels-Capital |
| Optimal Computing | |
| OPTIMIZE | Limburg |
| Optimizer S.P.R.L. | West Flanders |
| Optimus IT Services | |
| Optimy | |
| Option N.V. | Flemish Brabant |
| Oracle Belgium | Flemish Brabant |
| Orange Belgique | Brussels-Capital |
| Orange Fab Belux | Brussels-Capital |
| Orbit GeoSpatial Technologies N.V. | East Flanders |
| Ordiges S.A./N.V. | Brussels-Capital |
| Ordina | Antwerp |
| Orditech S.A. | Hainaut |
| Organi | Antwerp |
| Ortus B.V.B.A. | East Flanders |
| OSG B.V.B.A. | Antwerp |