| Nrh Electric | Nathan Rand Hillman | — | (707) 207 9637 | Electricians | Vacaville · Solano County | — | — |
| Nrj General Construction | Nrj General Construction | — | (760) 609-6099 | GC, remodel, ADU | Indio · Solano County | — | — |
| Ns Sewer Piping INC | Noe Hernan Sola | — | (707) 290 4547 | Other | Vallejo · Solano County | — | — |
| Nutt Refrigeration | Nutt Refrigeration | — | (707) 365 1416 | HVAC | Fairfield · Solano County | — | — |
| O M C Construction | Nunez Homar | omcconstruction1@gmail.com | (267) 504-3777 | GC, remodel, ADU | Vallejo · Solano County | — | — |
| O M H Landscaping INC | Higuera Osiel Molina | — | (707) 863 2844 | Windows, patio, painters | Fairfield · Solano County | — | — |
| O P Soluction Construction INC | Jose Luis Ortiz Zepeda | — | (707) 759 8785 | GC, remodel, ADU | Fairfield · Solano County | — | — |
| O Vargas Construction INC | Armenta Omar Vargas | — | (510) 812 7818 | GC, remodel, ADU | Vallejo · Solano County | — | — |
| O'brien Construction | Michael Gary O'brien | — | (408) 623 5169 | GC, remodel, ADU | Vallejo · Solano County | — | — |
| Oak Plumbing INC | Matthew Scott Harris | — | (707) 448 3567 | Other | Benicia · Solano County | — | — |
| Oak Street Management INC | Bernice Newell Robertson | — | (707) 640 9555 | Electricians | Vallejo · Solano County | — | — |
| Oakbrook Electric | Joel David Javier | — | (707) 704 1284 | Electricians | Fairfield · Solano County | — | — |
| Obrestad Landscaping | Obrestad Landscaping | — | (510) 612 1998 | GC, remodel, ADU | Fairfield · Solano County | — | — |
| Ocampo's Construction | Ocampo Camargo Mario | — | (510) 424 6085 | GC, remodel, ADU | Vallejo · Solano County | — | — |
| Og Electric | Gutierrez-arreola Oswaldo | — | (510) 778 3344 | Electricians | Vallejo · Solano County | — | — |
| Og Suppression | Og Suppression | — | (707) 249 0325 | Other | Benicia · Solano County | — | — |
| Ohana Remodeling | Corey Joseph Cruz | — | (707) 207 5490 | GC, remodel, ADU | Fairfield · Solano County | — | — |
| Ohm Pros Construction And Electric Corp | Justin Tyme Stimson | — | (707) 849 8413 | Electricians | Vallejo · Solano County | — | — |
| Old Time Landscaping | Old Time Landscaping | — | (707) 451 1025 | Windows, patio, painters | Vacaville · Solano County | — | — |
| Old West Iron INC | William Marshall Flagg | — | (707) 724 0033 | Other | Vacaville · Solano County | — | — |
| Omega Construction & Handyman Services INC | Jeremiah Tzun | — | (707) 333 4766 | GC, remodel, ADU | Vallejo · Solano County | — | — |
| Omni Energy Solutions INC | Michael Felix Del Valle | — | (707) 367 3448 | Electricians | Vacaville · Solano County | — | — |
| Omnia Builders | Raquel Keller | — | (707) 318 9273 | GC, remodel, ADU | Vallejo · Solano County | — | — |
| On Site Solution Construction Company | Carlos Rafael Hernandez-Lopez | — | (707) 334 4585 | GC, remodel, ADU | Vallejo · Solano County | — | — |
| On This Rock Construction INC | Sean Patrick Macdonald | — | (510) 493 5807 | GC, remodel, ADU | Vallejo · Solano County | — | — |
| Ondeck Acoustics INC | Tyler Christopher Ondeck | — | (510) 589 6869 | GC, remodel, ADU | Dixon · Solano County | — | — |
| One Hour Heating & Air Conditioning | Christopher Thomas George | — | (707) 816 9347 | HVAC | Vallejo · Solano County | — | — |
| One Love Construction | Keegan Russell Felker | — | (707) 321 7944 | GC, remodel, ADU | Vallejo · Solano County | — | — |
| One Man Plumbing | Joshua Paul Davis | — | (707) 439 6737 | Other | Rio Vista · Solano County | — | — |
| One Solution Plumbing & Drain | Juan Carlos Torres | — | (831) 905 7132 | Other | Rio Vista · Solano County | — | — |
| One Way Media Solutions INC | Masaki Liu | — | (707) 745 5052 | Other | Benicia · Solano County | — | — |
| Onward Outdoor Solutions LLC | Lee Jerald North | — | (707) 249 0571 | Electricians | Benicia · Solano County | — | — |
| Op Metal Works INC | Lopez Rogelio Ortiz | op.metalwork@gmail.com | (925) 899-0295 | Other | Fairfield · Solano County | — | — |
| Options Painting & Construction | Ahmet Kevin Yeksek | — | (415) 420 5061 | GC, remodel, ADU | Fairfield · Solano County | — | — |
| Orea Elite Concrete INC | Juan Pablo Orea Vargas | — | (707) 372 0049 | Other | Fairfield · Solano County | — | — |
| Orlando Camacho Barbosa | Orlando Camacho Barbosa | — | (707) 309 1609 | GC, remodel, ADU | Vallejo · Solano County | — | — |
| Ormiston's | Mark Curtiss Ormiston | — | (707) 446 1722 | Other | Elmira · Solano County | — | — |
| Ornelas Grading & Paving | Ornelas Grading & Paving | — | (707) 514 9329 | Other | Vacaville · Solano County | — | — |
| Oscar Lopez | Oscar Lopez | — | (707) 974 6481 | GC, remodel, ADU | Vacaville · Solano County | — | — |
| Owner Builder By You | Owner Builder By You | jim@ownerbuilderbyyou.com | (707) 363 2673 | GC, remodel, ADU | Fairfield · Solano County | — | — |