| Ogee Millwork LLC | Andrew Bogeski Franklin | andrew@ogeemillwork.com | (415) 243 5872 | GC, remodel, ADU | Richmond · Contra Costa County | — | — |
| Ohlson Construction Services INC | Carl Conrad Ohlson | — | (925) 408 9652 | GC, remodel, ADU | Clayton · Contra Costa County | — | — |
| Ohms Electric | Drew Hunter Michels | info@ohms-electric.com | (805) 636-2774 | Electricians | Concord · Contra Costa County | — | — |
| Ojeda's | Joseph Barry Ojeda | — | (510) 410 9381 | GC, remodel, ADU | Pinole · Contra Costa County | — | — |
| Old School Innovation Flooring INC | Cervantes Roberto Verduzco | — | (925) 237 0481 | Other | Concord · Contra Costa County | — | — |
| Old Town Contractors INC | Guzman Geronimo Mercado | — | (925) 978 4958 | Other roofers | Concord · Contra Costa County | — | — |
| Old Town Electrical INC | Ovidio Gonzalo Tobar Cruz | — | (510) 527 9517 | Electricians | Richmond · Contra Costa County | — | — |
| Oleksii Koziurenko | Oleksii Koziurenko | — | (925) 546 8810 | HVAC | Concord · Contra Costa County | — | — |
| Oleson Development | Oleson Development | — | (925) 580 7727 | GC, remodel, ADU | Bethel Island · Contra Costa County | — | — |
| Olivarez Landscape | Olivarez Landscape | — | (925) 787 7323 | Windows, patio, painters | Brentwood · Contra Costa County | — | — |
| Olive Renovation INC | De Oliveira Henrique Carvalho | — | (925) 450 8577 | GC, remodel, ADU | Brentwood · Contra Costa County | — | — |
| Oliver & Company INC | Joshua Henri Oliver | james@oliver.com | (510) 412 9095 | GC, remodel, ADU | Richmond · Contra Costa County | — | — |
| Oliver Peter Thonet | Oliver Peter Thonet | — | (925) 360 3394 | GC, remodel, ADU | Martinez · Contra Costa County | — | — |
| Oliver Souza | Oliver Souza | — | (925) 228 1984 | GC, remodel, ADU | Martinez · Contra Costa County | — | — |
| Olivero Plumbing Company INC | David John Olivero | — | (510) 233 3511 | Other | El Cerrito · Contra Costa County | — | — |
| Olson Bros INC | Gordon Keith Olson | — | (510) 669 9211 | GC, remodel, ADU | El Sobrante · Contra Costa County | — | — |
| Olsson Plumbing Company | Ronald Francis Vincent | — | (510) 522 2442 | HVAC | Danville · Contra Costa County | — | — |
| Olufson Garden Design | Olufson Garden Design | jimolufson@comcast.net | (925) 788 1379 | Windows, patio, painters | Lafayette · Contra Costa County | — | — |
| Oly Landscaping Construction INC | Gonzalez Olegario | — | (925) 706 7978 | Windows, patio, painters | Pittsburg · Contra Costa County | — | — |
| Omar Tijero Construction | Omar Javier Tijero | — | (925) 518 0837 | GC, remodel, ADU | San Ramon · Contra Costa County | — | — |
| Omar's Fences LLC | Dorix Omar Hernandez | info@omarsfences.com | (415) 342 2855 | GC, remodel, ADU | Richmond · Contra Costa County | — | — |
| Omega Blinds & Shutters | Isaac Obiora Dikeocha | — | (415) 867 8421 | Other | Antioch · Contra Costa County | — | — |
| Omega Paver Services INC | Patricia Oliveira Rezende | — | (510) 776 5213 | Other | El Sobrante · Contra Costa County | — | — |
| Omni Alarm Solutions INC | Eric Odin Billings | — | (925) 798 9550 | Electricians | Concord · Contra Costa County | — | — |
| Omni Painting And Waterproofing INC | Yilmaz Omer | — | (510) 654 3339 | GC, remodel, ADU | Kensington · Contra Costa County | — | — |
| On A Roll Paver Service | Magda Maria Boff | — | (650) 278 0231 | Windows, patio, painters | Concord · Contra Costa County | — | — |
| On Point Builders General Contractor | On Point Builders General Contractor | — | (415) 240 9134 | GC, remodel, ADU | San Pablo · Contra Costa County | — | — |
| On Point Building Services | Christian Allan Lewis | — | (650) 743 6896 | GC, remodel, ADU | Point Richmond · Contra Costa County | — | — |
| On Site Wood Wizard INC | Darrell Arthur Stimson | wizard@onsitewoodwizard.com | (925) 370 1489 | Other | Martinez · Contra Costa County | — | — |
| On The Rock Construction INC | Ordunez Martin Bravo | — | (415) 699 4192 | GC, remodel, ADU | Oakley · Contra Costa County | — | — |
| On Top Construction | Mohammad Kesser Jawed | — | (925) 437 5569 | GC, remodel, ADU | Oakley · Contra Costa County | — | — |
| On Track Remodeling | Aldo Alfredo Aranda | — | (510) 333 6211 | GC, remodel, ADU | Orinda · Contra Costa County | — | — |
| One & Only Electrical Service INC | Sabag Moshe | — | (925) 989 5266 | Electricians | Walnut Creek · Contra Costa County | — | — |
| One Hour Heating And Air Conditioning Of Pleasant Hill | Christopher Gerard Frye | — | (925) 323 7402 | HVAC | Pleasant Hill · Contra Costa County | — | — |
| One Stop Remodel And Building | Geoffrey Laurence Flanagan | — | (925) 330 9301 | GC, remodel, ADU | Concord · Contra Costa County | — | — |
| One Stop Shop Construction | Oscar Antonio Salas | — | (925) 360 5442 | GC, remodel, ADU | Martinez · Contra Costa County | — | — |
| Onesource Cleaning Construction Clean Up | Anthony Ray Jackson | — | (510) 872 0776 | Other | El Cerrito · Contra Costa County | — | — |
| Onnit Electric INC | Oscar Raul Jr Marquez | — | (925) 595 2380 | Electricians | Antioch · Contra Costa County | — | — |
| Onsite Painting And Decorating | Onsite Painting And Decorating | — | (925) 324 3702 | Windows, patio, painters | Concord · Contra Costa County | — | — |
| Ony Homes General Contractors LLC | Mark Leonidovich Chernomordikov | — | (925) 360 2889 | GC, remodel, ADU | Alamo · Contra Costa County | — | — |