| Ojai Custom Paint INC DBA Titus Painting | Scott Gilbert Titus | — | (805) 646 1373 | Windows, patio, painters | Ojai · Ventura County | — | — |
| Ojai Electric LLC | George Joel Hartmann | — | (805) 640 1011 | Electricians | Ojai · Ventura County | — | — |
| Ojai Pool Store | Troy Breton Becker | — | (805) 640 1288 | GC, remodel, ADU | Ojai · Ventura County | — | — |
| Ojai Rooter & Plumbing INC | Hugo Manuel Garcia | — | (805) 628 6698 | Other | Ojai · Ventura County | — | — |
| Ojai Tree Management | Courtland James King | — | (425) 512 5912 | Other | Ojai · Ventura County | — | — |
| Ojai Valley Flooring INC | Shawn Phillip Bones | — | (805) 670 0111 | Other | Oak View · Ventura County | — | — |
| Olivas Pools | Kirk Roland Olivas | — | (805) 207 9167 | GC, remodel, ADU | Simi Valley · Ventura County | — | — |
| Olmos Contractors Inc. | Vaca Ismael Olmos | — | (805) 760 8333 | GC, remodel, ADU | Ventura · Ventura County | — | — |
| Olsen And Son's Construction Co | Paul Dean Olsen | — | (805) 573 4000 | GC, remodel, ADU | Newbury Park · Ventura County | — | — |
| Olson Construction | Wesley Scott Olson | — | (805) 223 1878 | GC, remodel, ADU | Oak View · Ventura County | — | — |
| Olvera Plumbing And Rooter | Olvera Plumbing And Rooter | — | (805) 400 9817 | Other | Simi Valley · Ventura County | — | — |
| Olympic Electric INC | Juan Ramirez | — | (805) 791 7819 | Electricians | Simi Valley · Ventura County | — | — |
| Olympos Construction INC | Nicolaos Kkolis | — | (818) 904 0662 | GC, remodel, ADU | Simi Valley · Ventura County | — | — |
| Omid Construction | Hendessi Omid | — | (818) 577 7400 | GC, remodel, ADU | Westlake Village · Ventura County | — | — |
| Omnibus Construction INC | Tae Un Kang | — | (310) 963 1182 | GC, remodel, ADU | Westlake Village · Ventura County | — | — |
| On Point Pool & Spa | On Point Pool & Spa | — | (805) 405 4132 | Pool | Thousand Oaks · Ventura County | — | — |
| On Point Striping | On Point Striping | — | (805) 727 0548 | Other | Santa Paula · Ventura County | — | — |
| On The Level Construction | Randy Martin Korop | — | (661) 400 2504 | GC, remodel, ADU | Ventura · Ventura County | — | — |
| On The Mark Creative INC | Mark Andrew Collis | — | (818) 294 1000 | Other | Westlake Village · Ventura County | — | — |
| On Time Electric | Jeff Gordon Kaye | — | (805) 794 8596 | Electricians | Thousand Oaks · Ventura County | — | — |
| On Time Water Heaters INC | Joshua Richard Santor | — | (844) 446 8987 | Other | Simi Valley · Ventura County | — | — |
| One King Fire Protection | One King Fire Protection | — | (805) 814 6133 | Other | Ventura · Ventura County | — | — |
| One Pool At A Time Pool Equipment Repair Specialist | Christopher Matthew Curoe | — | (818) 451 8394 | Pool | Simi Valley · Ventura County | — | — |
| One Stop H V A C | Omar Garcia Mejia | — | (818) 605 5439 | HVAC | Simi Valley · Ventura County | — | — |
| One Stop Home Construction | Jimmy Dean De Clercq | — | (805) 279 3964 | GC, remodel, ADU | Simi Valley · Ventura County | — | — |
| One Stop Water INC | Daniel Sandor Harasty | — | (805) 642 4188 | Other | Ventura · Ventura County | — | — |
| Oneplus Integrations LLC | Jose Luis Contreras | — | (805) 509 2826 | Other | Oxnard · Ventura County | — | — |
| Onsite Builder Creations | Michael Scott Campbell | — | (805) 432 5806 | GC, remodel, ADU | Simi Valley · Ventura County | — | — |
| Op Coastal Construction INC | Flores Agustin Ocampo | — | (805) 366 5418 | GC, remodel, ADU | Oxnard · Ventura County | — | — |
| Optimal Technology Well Drilling | Adam Samuel Brockman | — | (818) 734 6230 | Other | Thousand Oaks · Ventura County | — | — |
| Optimized Paint Management INC | Esteban Angel Ortiz | — | (805) 708 3969 | Windows, patio, painters | Oxnard · Ventura County | — | — |
| Optimum Energy Services | Steve Andrew Smay | — | (805) 529 5682 | Electricians | Westlake Village · Ventura County | — | — |
| Ordonez Plumbing | Ordonez Plumbing | — | (805) 696 3789 | Other | Ventura · Ventura County | — | — |
| Ordonez Rain Gutters | Ordonez Rain Gutters | — | (805) 338 8154 | Other | Simi Valley · Ventura County | — | — |
| Originedge Pros | Originedge Pros | — | (805) 269-9270 | Restoration | Ventura · Ventura County | — | — |
| Orlando Construction & Development Company | Thomas Jerard Orlando | — | (805) 794 3941 | GC, remodel, ADU | Camarillo · Ventura County | — | — |
| Orozco & Sons Painting | Javier Orozco | — | (805) 444 1072 | Windows, patio, painters | Oxnard · Ventura County | — | — |
| Orozco Plastering | Orozco Plastering | — | (805) 881 2739 | Other | Oxnard · Ventura County | — | — |
| Ortiz & Sons Builders INC | Martinez Juan Espinosa | — | (805) 625 0083 | GC, remodel, ADU | Oxnard · Ventura County | — | — |
| Osborne Construction Management LLC | Brian James Osborne | — | (917) 586 2098 | GC, remodel, ADU | Ventura · Ventura County | — | — |