 | OPTIHUS AS | 920811493 | HORTEN |
| OPTIMAL BRANNSIKKERHET AS | 929261879 | SKIEN |
| OPTIMAL ENTREPRENØR AS | 999169449 | FREDRIKSTAD |
| OPTIMAL RØRSERVICE AS | 919329114 | FROGN |
| OPTIMERA AS | 967013056 | OSLO |
| OPTIMERA BYGGSYSTEMER AS | 979658125 | OSLO |
| OPUS BERGEN AS | 921610602 | BERGEN |
| OPV CONSULTING AS | 917880697 | OSLO |
| ØRA ELEKTRO AS | 827026042 | SUNNDAL |
| ØRA RØR AS | 929473698 | SUNNDAL |
| ORIGO ARKITEKTGRUPPE AS | 943577374 | BERGEN |
| ORIGON AS | 933374580 | BÆRUM |
| ORION VAKTMESTERSERVICE AS | 983074200 | TRONDHEIM |
| ORKDAL VARME OG SANITÆR AS | 980557413 | ORKLAND |
| ORKLA BLIKK & VENTILASJON AS | 993794996 | ORKLAND |
| ORKLA LAFT AS | 986471219 | ORKLAND |
| ORKLA MARDAHL AS | 913684281 | ORKLAND |
| ORKLAND-BYGGTJENESTER AS | 930878383 | ORKLAND |
| ORKLAND KLIMA- OG KULDETEKNIKK AS | 922284032 | ORKLAND |
| ORKLAND VVS AS | 922822174 | ORKLAND |
| ORKLA STÅLKONSULT AS | 991235787 | ORKLAND |
| Ørnulf Wiig Installasjon A/S | 917339236 | BÆRUM |
| ORONA NORWAY AS | 984176503 | HAMAR |
| ORSTAD RØR AS | 989067621 | KLEPP |
| ØRSTA VEDLIKEHALD AS | 958339216 | ØRSTA |
| OSBYGG AS | 998256259 | BJØRNAFJORDEN |
| OSE AS | 994525174 | ØRSTA |
| OSE BYGG AS | 927409216 | KRISTIANSAND |
| OS ELEKTRO AS | 929975049 | BJØRNAFJORDEN |
| OS ENTREPRENØR AS | 986662707 | OS (INNLANDET) |
| OSKAR BRUGRAND AS | 974531178 | LÆRDAL |
| OSKAR JOHANSEN AS | 971082879 | NOTODDEN |
| OSKAR OG TORMOD WIKE AS | 930705462 | TØNSBERG |
| ØS KONSULT AS | 989193139 | ASKER |
| OSLAND MASKIN AS | 912309142 | KLEPP |
| OSLO-AKERSHUS RØRLEGGERBEDRIFT AS | 994920790 | OSLO |
| OSLO BAD AS | 995797267 | OSLO |
| OSLO BAD & FLIS AS | 931298739 | OSLO |
| OSLO BAD & RENOVERING AS | 813498952 | OSLO |
| OSLO BLIKKENSLAGERMESTER AS | 922541639 | OSLO |
| OSLO BYGGASSISTANSE AS | 916093632 | NORDRE FOLLO |
| OSLO BYGGENTREPRENØR AS | 950030119 | OSLO |
| OSLO-BYGGENTREPRENØR AS | 921331932 | ULLENSAKER |
| OSLO BYGGMESTER SERVICE DRIFT AS | 932691175 | SØR-ODAL |
| OSLO BYGG OG MONTASJESERVICE AS | 925015008 | OSLO |
| OSLO BYGGREHAB AS | 917016283 | OSLO |
| OSLO BYGG TEKNIKK AS | 914731224 | OSLO |
| OSLO ELECTRO COMPANY AS | 963983549 | OSLO |
| OSLO ELEKTRISKE AS | 961732778 | OSLO |
| OSLO ENTREPRENØR AS | 964676607 | OSLO |