| Nusa Solar | Badung, Indonesia | Visit site |
| Nusec GmbH | Beverungen, Germany | Visit site |
| Nutz GmbH | Ampfing, Germany | Visit site |
| Nuuko Power | Castries, Saint Lucia | Visit site |
| Nuuko Solar GmbH | Frankfurt am Main, Germany | Visit site |
| Nuvo Energy Africa | Johannesburg, South Africa | Visit site |
| Nxtech AS | Kråkerøy, Norway | Visit site |
| Nyobolt | Cambridge, United Kingdom | · |
| Núna Energy | Hilversum, Netherlands | Visit site |
| O'M Engineering | Vancouver, Canada | Visit site |
| O'sitoit, Membre fondateur de Wewise | Aiserey, France | · |
| O-INNOVATIONS | Lancaster, United Kingdom | Visit site |
| OCTANE Management Consultants | Athina, Greece | Visit site |
| ODTÜ-GÜNAM | Çankaya, Türkiye | Visit site |
| OE Industrial | Shanghai, China | · |
| OE-EN | Baar, Switzerland | Visit site |
| OEG | Aberdeen, United Kingdom | Visit site |
| OFGEN Africa | Nairobi, Kenya | Visit site |
| OHLA Group | Madrid, Spain | Visit site |
| OHM Energy Ltd | Hailsham, United Kingdom | Visit site |
| OHMS Energy Pvt. Ltd. | Mumbai, India | Visit site |
| OIE Srbija | Belgrade, Serbia | · |
| OIS Offshore Industrie Service GmbH | Rostock, Germany | Visit site |
| OLG | Cleethorpes, United Kingdom | Visit site |
| OLG | Immingham, United Kingdom | Visit site |
| OMNI Conversion Technologies | Kanata, Canada | Visit site |
| OMS Photovoltaik | Garbsen, Germany | Visit site |
| ON Battery B.V. | Oisterwijk, Netherlands | Visit site |
| ON SOLAR | Rwanda | Visit site |
| ONE TRIBE | Manchester, United Kingdom | Visit site |
| ONR Global Enerji | Istanbul, Türkiye | · |
| ONSOLAR ENERJİ | Ataşehir, Türkiye | Visit site |
| ONergy Solar | Kolkata, India | Visit site |
| ONiO | Oslo, Norway | Visit site |
| OOO "Kohlbach Bioenergetika" | Липецк, Russia | · |
| OPES Solar Mobility | Zwenkau, Germany | Visit site |
| OPTI-Solar | 中和區, Taiwan | Visit site |
| OPTIPUS PV | France | Visit site |
| OREC (Ottawa Renewable Energy Co-operative) | Ottawa, Canada | Visit site |
| OREX | Dubai, United Arab Emirates | Visit site |
| ORLEN Neptun | Warszawa, Poland | Visit site |
| ORTECO | Piledriver producers | Bologna, Italy | Visit site |
| ORTECO | Piledriver producers | Anzola dell'Emilia, Italy | Visit site |
| OSDA SOLAR | Ningbo Shi, China | Visit site |
| OSG Oriana India Pvt Ltd | Pune, India | Visit site |
| OSL Consulting Engineers | Hull, United Kingdom | Visit site |
| OWEMES | Roma, Italy | Visit site |
| OWS Off-Shore Wind Solutions GmbH | Emden, Germany | Visit site |
| OX2 | Stockholm, Sweden | Visit site |
| OXCCU TECH LTD | Oxford, United Kingdom | Visit site |
| OXLiD Ltd | Nottingham, United Kingdom | Visit site |
| Oasis Infrastructure Pvt. Ltd. | Mumbai, India | Visit site |
| Oasthouse Ventures Limited | Brighton, United Kingdom | Visit site |
| Obelisk Energy | Cape Town, South Africa | Visit site |
| Obelisk Energy | Lystrup, Denmark | Visit site |
| Oberon Energy Group | Madrid, Spain | · |
| Obsidian Engineering | Calgary, Canada | Visit site |
| Obton A/S | Aarhus, Denmark | Visit site |
| Obton Italy | Milano, Italy | Visit site |
| Ocean Ecology Ltd | Gloucester, United Kingdom | Visit site |
| Ocean Energy Europe | Brussels, Belgium | Visit site |
| Ocean Energy Pathway | London, United Kingdom | · |
| Ocean Grazer | Groningen, Netherlands | Visit site |
| Ocean Impact Organisation | Ourimbah, Australia | Visit site |
| Ocean Sun | Sharjah, United Arab Emirates | Visit site |
| OceanEnergy | Cobh, Ireland | Visit site |
| Octago Advisory Solutions LLP | Mumbai, India | Visit site |
| Octave | Mechelen, Belgium | · |
| Octopus Energy Generation | London, United Kingdom | Visit site |
| Octopus Investments | London, United Kingdom | Visit site |
| Odeskabel | Одеса, Ukraine | Visit site |
| Odfjell Oceanwind | Kokstad, Norway | Visit site |
| Odfjell Oceanwind | Tananger, Norway | Visit site |
| Odfjell Oceanwind | Aberdeen, United Kingdom | Visit site |
| Odfjell Oceanwind | Oslo, Norway | Visit site |
| Odqa Renewable Energy Technologies | Oxford, United Kingdom | · |
| Off Grid | Dilbeek, Belgium | Visit site |
| Off-Grid Energy Australia | Adelaide, Australia | Visit site |
| Off-Grid Europe | Pfullendorf, Germany | Visit site |
| OffGrid Power Wind Limited | Northampton, United Kingdom | Visit site |
| Offgridsun | Cittadella, Italy | Visit site |
| Offshore Competence Thorhorst GmbH | Lamstedt, Germany | Visit site |
| Offshore Energies UK | London, United Kingdom | Visit site |
| Offshore Renewable Energy Catapult | Glasgow, United Kingdom | · |
| Offshore Wind Foundations Alliance (OWFA) | Brussels, Belgium | Visit site |
| Offshore Wind Growth Partnership | Glasgow, United Kingdom | · |
| OffshoreChannel | Türkiye | Visit site |
| Olin Enerji | Çankaya, Türkiye | Visit site |
| Oliveira Trust | Rio de Janeiro, Brazil | Visit site |
| Om Solar Solutions | Lucknow, India | Visit site |
| Om Solar Solutions | Mahendragarh, India | Visit site |
| Om Solar Solutions | Chhibramau, India | Visit site |
| Om Solar Solutions | Prayagraj, India | Visit site |
| Om Solar Solutions | Kanpur, India | Visit site |
| Oman Solar Beam | مَسْقَط, Oman | Visit site |
| Omega Sonnen | Perai, Malaysia | Visit site |
| Omnes | Paris, France | Visit site |
| Omnes | Brussels, Belgium | Visit site |
| Omnes | München, Germany | Visit site |
| Omnes | Zürich, Switzerland | Visit site |
| Omnicon | Cali, Colombia | Visit site |
| Omtech Electricals Solutions Pvt. Ltd. | Pune, India | Visit site |
| Omtech Electricals Solutions Pvt. Ltd. | Bengaluru, India | Visit site |
| OnPath Energy | Houghton-le-Spring, United Kingdom | Visit site |
| OnRope | Porto, Portugal | Visit site |
| Onasolar | Qena, Egypt | Visit site |
| Ondulit Italiana | Fiumicino, Italy | Visit site |
| One Energy | Burnley, United Kingdom | Visit site |
| One Hub Energy | Barcelona, Spain | Visit site |
| One Planet Developments Limited | Wimborne, United Kingdom | Visit site |
| One Power Solar | Gold Coast, Australia | Visit site |
| One Power Solar | Port MacDonnell, Australia | Visit site |
| One Power Solar | Newcastle, Australia | Visit site |
| One Wind | Dartmouth, Canada | Visit site |
| One Zone | Hong Kong, Hong Kong | Visit site |
| One Zone | Manila, Philippines | Visit site |
| One Zone | United Kingdom | Visit site |
| One Zone | Tullyvin, Ireland | Visit site |
| One-Inverter | Fuoshan Shi, China | Visit site |
| OnePower | Maseru, Lesotho | Visit site |
| OneSolar International GmbH | Eching, Germany | Visit site |
| Onitl | Yangzhou Shi, China | Visit site |
| Onix Renewable | Lodhika, India | Visit site |
| Onken Offshore Contractors GmbH | Nordenham, Germany | Visit site |
| OnlySun.fr | Lyon, France | Visit site |
| Ontario Energy Board | Toronto, Canada | Visit site |
| Ontario Power Generation | Toronto, Canada | Visit site |
| Ontario Teachers' Pension Plan | Toronto, Canada | Visit site |
| Onur Group | Çankaya, Türkiye | Visit site |
| Onur Group | Türkiye | Visit site |
| Onur Group | Ukraine | Visit site |
| Onyx Solar | Ávila, Spain | Visit site |
| Ooitech | China | Visit site |
| Oolu | Dakar, Senegal | Visit site |
| Oomi Solar | Vantaa, Finland | Visit site |
| Oorjan Cleantech | Mumbai, India | Visit site |
| Oort Energy | London, United Kingdom | · |
| Op Naar Nul | Arnhem, Netherlands | Visit site |
| Opea | São Paulo, Brazil | Visit site |
| OpenCaps | Lyon, France | · |
| OpenStar Technologies | Wellington, New Zealand | Visit site |
| Operadores Nacionales | Guatemala, Guatemala | Visit site |
| Operadores Nacionales | Santa Catarina Pinula, Guatemala | Visit site |
| Opt Energy | Melbourne, Australia | Visit site |
| Optec AG | Wetzikon, Switzerland | Visit site |
| Opteco | Zonhoven, Belgium | Visit site |
| OptiMAXX | Bengaluru, India | Visit site |
| Optimize Energy | Madrid, Spain | Visit site |
| Optimum Sun | Xiamen Shi, China | Visit site |
| Optimus Energy (HERON | GEK TERNA Group) | Athina, Greece | Visit site |