| Op Pressure Washing | Jacob Conner Oliker | — | (925) 389 1515 | Other | Danville · Contra Costa County | — | — |
| Opalprop Construction INC | Mario Ibarra Tenorio | — | (925) 819 3873 | GC, remodel, ADU | San Ramon · Contra Costa County | — | — |
| Opening Technologies INC | Nathan Alan Burkhardt | — | (925) 674 6900 | GC, remodel, ADU | Pittsburg · Contra Costa County | — | — |
| Operational Technology Integrators | Bridget Anne Davies | — | (925) 948 8476 | Other | Concord · Contra Costa County | — | — |
| Optimiz3 | Jordan Christopher Reise | — | (415) 858 7230 | GC, remodel, ADU | Richmond · Contra Costa County | — | — |
| Optimum Flooring | Optimum Flooring | — | (925) 459 4373 | Other | Oakley · Contra Costa County | — | — |
| Optimum Tilework INC | Garcia Miguel Moreno | — | (619) 797 0633 | Other | Brentwood · Contra Costa County | — | — |
| Optivest Build INC | Asl Mohammad Rahmani | info@optivestbuild.com | (979) 739 5956 | GC, remodel, ADU | Walnut Creek · Contra Costa County | — | — |
| Opulens Construction INC | Juan Ramon Fierro Pelayo | — | (510) 369 4019 | GC, remodel, ADU | Richmond · Contra Costa County | — | — |
| Oqp Corporation | Sarbelio Eliazar Hernandez Recinos | — | (415) 246 5761 | Windows, patio, painters | Richmond · Contra Costa County | — | — |
| Ordaz General Marble & Granite | Francisco Javier Aria Ordaz | — | (925) 671 9639 | Other | Concord · Contra Costa County | — | — |
| Orfali Construction | John Paul Orfali | — | (510) 219 1070 | Electricians | Richmond · Contra Costa County | — | — |
| Origin Mechanical & HVAC INC | Taylor G Andrews | — | (510) 387 9233 | HVAC | Pleasant Hill · Contra Costa County | — | — |
| Orlando Construction | Richard Pierre Orlando | — | (925) 459 4640 | GC, remodel, ADU | Martinez · Contra Costa County | — | — |
| Orman & Associates INC | Autumn Elizabeth Orman | — | (925) 577 8030 | GC, remodel, ADU | Alamo · Contra Costa County | — | — |
| Ornamental Metal Design | Ornamental Metal Design | — | (925) 265 3652 | Other | Concord · Contra Costa County | — | — |
| Ornelas Plumbing INC | Angel Abran Ornelas | — | (925) 250 6835 | Other | Pittsburg · Contra Costa County | — | — |
| Orocstone LLC | Stanislav Pokazanev | — | (925) 642 0620 | GC, remodel, ADU | Antioch · Contra Costa County | — | — |
| Orozco And Sons Concrete LLC | Arci Quintana Orozco | — | (925) 978 3955 | Other | Antioch · Contra Costa County | — | — |
| Orozco Tile And Granite | Fernando Orozco Flores | — | (510) 691 8520 | Other | Hercules · Contra Costa County | — | — |
| Orta & Sons Painting Services Incorporated | Ramon De Jesus Orta-De La Mora | — | (925) 439 8942 | Windows, patio, painters | Pittsburg · Contra Costa County | — | — |
| Orta Construction & Renovation INC | Jose Manuel Caminero | — | (925) 586 6263 | GC, remodel, ADU | Pittsburg · Contra Costa County | — | — |
| Orta Legacy Painting INC | Arturo Orta | — | (925) 586 4509 | Windows, patio, painters | Brentwood · Contra Costa County | — | — |
| Ortega Carpentry | Ortega Carpentry | — | (925) 383 8716 | Other | Concord · Contra Costa County | — | — |
| Ortega Construction | Juan Carlos Ortega | — | (510) 435 4520 | GC, remodel, ADU | Richmond · Contra Costa County | — | — |
| Ortega Plumbing & Rooter LLC | Ortega-acosta Pedro | — | (510) 502 6937 | Other | Richmond · Contra Costa County | — | — |
| Ortega Structural Frame INC | Leonel Zarate-Ortega | — | (925) 383 8716 | Other | Concord · Contra Costa County | — | — |
| Ortiz General Contractor INC | Jimmy Phan | — | (510) 563 9933 | GC, remodel, ADU | Hercules · Contra Costa County | — | — |
| Ortiz Tile | Ortiz Tile | — | (510) 685 8270 | Other | Richmond · Contra Costa County | — | — |
| Os1 Construction INC | Jose V Osuna | — | (510) 820 4820 | Other roofers | Richmond · Contra Costa County | — | — |
| Oscar Hoyos Construction | Oscar Mauricio Hoyos | — | (415) 265 3306 | GC, remodel, ADU | Richmond · Contra Costa County | — | — |
| Oscar Javier Solano Bermudez | Oscar Javier Solano Bermudez | — | (510) 395 2895 | HVAC | El Sobrante · Contra Costa County | — | — |
| Oscar Landscaping INC | Oscar Antonio Hernandez | — | (925) 597 1013 | GC, remodel, ADU | Alamo · Contra Costa County | — | — |
| Oscar Mejia Painting | Oscar Mejia Painting | — | (415) 646 6607 | Windows, patio, painters | Richmond · Contra Costa County | — | — |
| Oseguera Mario Construction | Oseguera Mario | — | (925) 260 5467 | GC, remodel, ADU | Oakley · Contra Costa County | — | — |
| Oso Construction | Mario Anthony Anicama | — | (510) 356 6808 | GC, remodel, ADU | Pinole · Contra Costa County | — | — |
| Otis Construction INC | Brian Mathew Otis | — | (925) 308 0997 | GC, remodel, ADU | Byron · Contra Costa County | — | — |
| Otter Services HVAC LLC | Alibi Kidiraliyev | — | (925) 326 9171 | HVAC | Pleasant Hill · Contra Costa County | — | — |
| Otto & Sons Construction | Alani Petelo Tangitau | — | (925) 435 4516 | GC, remodel, ADU | Antioch · Contra Costa County | — | — |
| Out Of Ordinary Development Construction INC | Changiz Talamroud | — | (831) 821 0529 | GC, remodel, ADU | Walnut Creek · Contra Costa County | — | — |