| OCI Associates | Maitland, Florida | Visit site |
| OCI Energy | San Antonio, Texas | Visit site |
| OCTANE Management Consultants | Athina, Attica | Visit site |
| OE Inc. | Grand Rapids, Michigan | Visit site |
| OEL Worldwide Industries | Monument, Colorado | Visit site |
| OHLA Group | Madrid, Madrid | Visit site |
| OKANDAN CAM SAN.TIC.A.S. | Kayseri, Melikgazi | Visit site |
| OLG | Cleethorpes, England | Visit site |
| OMCO Solar | Phoenix, Arizona | Visit site |
| OMERS | Toronto, Ontario | Visit site |
| OMG Roofing Products | Agawam, Massachusetts | Visit site |
| OMRON Group | 京都市, Kyoto | Visit site |
| ONE TRIBE | Manchester, England | Visit site |
| ONTILITY | Houston, Texas | Visit site |
| ONYX Engineering | Holladay, Utah | Visit site |
| ONiO | Oslo, Oslo | Visit site |
| ORPC | Portland, Maine | Visit site |
| ORR Protection | Louisville, KY | Visit site |
| ORTECO | Piledriver producers | Bologna, Bologna | Visit site |
| OSL Consulting Engineers | Hull, England | Visit site |
| OTT HydroMet (OHM) | · | Visit site |
| OWEMES | Roma, Rome | Visit site |
| Oakley Services | Atlanta, Georgia | Visit site |
| Oasis Infrastructure Pvt. Ltd. | Mumbai, Maharashtra | Visit site |
| Oasthouse Ventures Limited | Brighton, England | Visit site |
| Obelisk Energy | Cape Town, Western Cape | Visit site |
| Obsidian Engineering | Calgary, Alberta | Visit site |
| Obsidian Renewables, LLC | Lake Oswego, Oregon | Visit site |
| Obvious Ventures | San Francisco, California | Visit site |
| Ocean Ecology Ltd | Gloucester, England | Visit site |
| Ocean Energy Europe | Brussels, Brussels-Capital | Visit site |
| Ocean Impact Organisation | Ourimbah, New South Wales | Visit site |
| Ocean Infinity | Austin, Texas | Visit site |
| Ocean Motion Technologies | San Diego, California | Visit site |
| Ocean Sun | Sharjah, Sharjah | Visit site |
| OceanBit | Honolulu, Hawaii | Visit site |
| Octago Advisory Solutions LLP | Mumbai, Maharashtra | Visit site |
| Octopus Energy Generation | London, England | Visit site |
| Octopus Impact Investment Group Inc. | Toronto, Ontario | Visit site |
| Octopus Investments | London, England | Visit site |
| Odeskabel | Одеса, Odesa Oblast | Visit site |
| Odfjell Oceanwind | Kokstad, Vestland | Visit site |
| Offshore Energies UK | London, England | Visit site |
| Offshore Wind Foundations Alliance (OWFA) | Brussels, Brussels-Capital | Visit site |
| Oklo Inc | Santa Clara, California | Visit site |
| Okonite | Ramsey, New Jersey | Visit site |
| Oldcastle Infrastructure | Atlanta, Georgia | Visit site |
| Oliveira Trust | Rio de Janeiro, Rio de Janeiro | Visit site |
| Olsen-Beal Associates | Lindon, Utah | Visit site |
| Olsson | Tecumseh, Nebraska | Visit site |
| Om Solar Solutions | Lucknow, Uttar Pradesh | Visit site |
| Omega Solar | Little Rock, Arkansas | Visit site |
| Omega Sonnen | Perai, Penang | Visit site |
| Omnes | Paris, Île-de-France | Visit site |
| Omni Power | Glendale, Arizona | Visit site |
| Omni2Controls | Santa Rosa Beach, Florida | Visit site |
| Omnicon | Cali, Valle del Cauca | Visit site |
| Omnis Power USA Inc | Spokane, Washington | Visit site |
| Omnitron Sensors | Montebello, California | Visit site |
| Omron Automation | Hoffman Estates, Illinois | Visit site |
| Omtech Electricals Solutions Pvt. Ltd. | Pune, Maharashtra | Visit site |
| OnSight Technology | Thermal, California | Visit site |
| Ondulit Italiana | Fiumicino, Rome | Visit site |
| One Power Solar | Gold Coast, Queensland | Visit site |
| One Source Freight Solutions | Phoenix, Arizona | Visit site |
| One Zone | Hong Kong, Hong Kong | Visit site |
| OneD Battery Sciences | Palo Alto, California | Visit site |
| OneEthos | St. Petersburg, Florida | Visit site |
| OneMonroe Titan | Rochester Hills, Michigan | Visit site |
| OnePower | Maseru, Maseru | Visit site |
| OneRoof Solution | Visalia, California | Visit site |
| OneSource Restoration | New Market, Iowa | Visit site |
| Onitl | Yangzhou Shi, Jiangsu | Visit site |
| Onsite Solar, LLC. | Liverpool, New York | Visit site |
| Ontario Energy Board | Toronto, Ontario | Visit site |
| Ontario Power Generation | Toronto, Ontario | Visit site |
| Ontario Teachers' Pension Plan | Toronto, Ontario | Visit site |
| Onur Group | Çankaya, Ankara | Visit site |
| Onyx Solar | Ávila, Ávila | Visit site |
| Op Naar Nul | Arnhem, Gelderland | Visit site |
| Opea | São Paulo, São Paulo | Visit site |
| OpenSolar | Sydney, New South Wales | Visit site |
| OpenStar Technologies | Wellington, Wellington | Visit site |
| Operadores Nacionales | Guatemala, Guatemala | Visit site |
| Operant Networks | Rohnert Park, California | Visit site |
| Opportunity Finance Network | Washington, District of Columbia | Visit site |
| Opsun Systems Inc. | Québec, Quebec | Visit site |
| Optec AG | Wetzikon, Zürich | Visit site |
| Opteco | Zonhoven, Limburg | Visit site |
| OptiMAXX | Bengaluru, Karnataka | Visit site |
| Optigon | Somerville, Massachusetts | Visit site |
| Optimize Energy | Madrid, Madrid | Visit site |
| Optimotive | Windsor, Ontario | Visit site |
| Optimum Academy | Amarillo, Texas | Visit site |
| Optimum EPC Contracting | Portland, Maine | Visit site |
| Optimum Rope Access Solutions | Amarillo, Texas | Visit site |
| Optimum Solar Power Inc. | Corona, California | Visit site |
| Optimum Tracker | Provence-Alpes-Côte d'Azur | Visit site |
| Option One Solar | Apple Valley, California | Visit site |
| Optivolt | San Francisco, California | Visit site |
| Opulent Power Solutions, LLC. | Anaheim, California | Visit site |
| Orange Solar | Johannesburg, Gauteng | Visit site |
| Orano | Paris, Île-de-France | Visit site |
| Orascom Construction PLC | Cairo, Cairo | Visit site |
| Orbit Wires & Cables | Bell Gardens, California | Visit site |
| Orbital Engineering, Inc. | Pittsburgh, Pennsylvania | Visit site |
| Orchid Solar | Edinburgh, Scotland | · |
| Orennia | Calgary, Alberta | Visit site |
| Orion Power | Laguna Beach, California | Visit site |
| Orion Renewable Energy Group LLC | Oakland, California | Visit site |
| Oritia & Boreas | Granada, Granada | Visit site |
| Orka náttúrunnar / ON Power | Reykjavík, Capital Region | Visit site |
| Orkan Automação Industrial | São Bernardo do Campo, São Paulo | Visit site |
| Orkland Energi | Fannrem, Trøndelag | Visit site |
| Orkuveitan | Reykjavík, Capital Region | Visit site |
| Ormazabal | Derio, Biscay | Visit site |
| Osda Solar | Ningbo Shi, Zhejiang | Visit site |
| Oso Polar Energy | Móstoles, Madrid | Visit site |
| Otary RS NV | Oostende, West Flanders | Visit site |
| Ottawa Electrical Contractors' Association | Ottawa, Ontario | Visit site |
| Ottawa Solar Power | Ottawa, Ontario | Visit site |
| Our World Energy | Phoenix, Arizona | Visit site |
| Ourolux Iluminação e Energia Solar | Santos, São Paulo | Visit site |
| Oushang Photovoltaic | Shijiazhuang Shi, Hebei | Visit site |
| OutBack Power | New Braunfels, Texas | Visit site |
| Outsource Utility Contractor, LLC | Torrance, California | Visit site |
| Ovanova | Raleigh, North Carolina | Visit site |
| Over Easy Solar | Oslo, Oslo | Visit site |
| Overdrive Energy Solutions | Los Angeles, California | Visit site |
| Overland Contracting Inc. | Garner, North Carolina | Visit site |
| Overland Solar | Moses Lake, Washington | Visit site |
| Overlay Capital | Alpharetta, Georgia | Visit site |
| Oxera Consulting LLP | Oxford, England | Visit site |
| Oxford Sigma | Oxford, England | Visit site |
| Oze Grup | Ankara, Ankara | Visit site |
| Ozzies Inc | Phoenix, Arizona | Visit site |
| onCORE Origination | Watervliet, New York | Visit site |
| onsemi | Scottsdale, Arizona | Visit site |
| oursolarplan.co | Fort Lauderdale, Florida | Visit site |