| Onshus Bygg AS | Flatåsen | 2023 |
| Onsite Services AS | Tolvsrød | 2018 |
| Onstadvegen SA | Åmli | 1970 |
| Onsøien Maskin AS | Buvika | 2011 |
| Onsøy Rør AS | Engelsviken | 2019 |
| Onsøy Rørleggerbedrift AS | Gressvik | 1985 |
| Onsøy Skjærgårdservice AS | Gressvik | 2017 |
| Onsøyhus AS | Manstad | 2018 |
| Onyx Byggeservice AS | Trøgstad | 2025 |
| Onøy Maskinstasjon AS | Lurøy | 2019 |
| Ooc AS | Gol | 2014 |
| Ooo Kormet Estonia Branch | Ee-10613 Tallinn | 2018 |
| Oosloo AS | Oslo | 2022 |
| Op Bygg AS | Oslo | 2021 |
| Op Transport Og Graving AS | Ås | 2025 |
| Opa Glass AS | Nesttun | 1974 |
| Opa Glass Holding AS | Nesttun | 2020 |
| Opal Entreprenør AS | Nordfjordeid | 2006 |
| Opal Utbygging AS | Nordfjordeid | 2007 |
| Opcom AS | Dalsgrenda | 2018 |
| Opdahl Bygg & Rehab AS | Hønefoss | 2021 |
| Opdahl Og Pettersen Elektro AS | Lakselv | 1999 |
| Opdahl Solutions Ltd Nuf | Sortland | 2010 |
| Opedal Og Åkre Maskinservice AS | Lofthus | 2025 |
| Open House Production AB | Oslo | 2007 |
| Operamini AB Nuf | 53333 Götene | 2019 |
| Opg Anlegg AS | Hakadal | 2007 |
| Opgård AS | Bardufoss | 2010 |
| Opgård Gruppen AS | Alta | 2022 |
| Opgård Multiservice AS | Gullaug | 2025 |
| Oph Invest AS | Birkeland | 2018 |
| Opheim Malerservice AS | Trondheim | 2024 |
| Opheim Utbygging AS | Steinkjer | 2012 |
| Ophus Holding AS | Furnes | 2025 |
| Opland Invest AS | Lom | 2003 |
| Opland Maskin AS | Melhus | 2020 |
| Opnor AS | Storsteinnes | 2019 |
| Opp Kraftledningstjenester AS | Søgne | 2003 |
| Oppdal Bygg Holding AS | Oppdal | 2022 |
| Oppdal Graving Og Transport AS | Oppdal | 2013 |
| Oppdal Maskinkompani AS | Oppdal | 1969 |
| Oppdal Rørservice AS | Lønset | 2006 |
| Oppdal VVS A/S | Oppdal | 1988 |
| Oppdalslaft AS | Oppdal | 2007 |
| Oppedal Kapital AS | Stryn | 2023 |
| Oppegaard Mur Og Tradisjonshåndverk AS | Tvedestrand | 2024 |
| Oppegaard Varme- Og Kuldeservice AS | Vestre Gausdal | 2012 |
| Oppegård Elektriske AS | Oppegård | 2011 |
| Oppent S.P.A. | It-20876 Ornago (Mb) | 2021 |
| Oppgradert Bad AS | Hamar | 2023 |
| Oppheim Gravemaskinstasjon AS | Vossestrand | 1995 |
| Oppheimjordet AS | Bø I Telemark | 2014 |
| Oppkjøp AS | Vinstra | 2014 |
| Oppkoma AS | Stol | 2007 |
| Oppkomme AS | Nesbru | 2013 |
| Oppland Bygg & Anlegg AS | Brandbu | 1990 |
| Oppland Bygg Og Vedlikehold AS | Hunndalen | 2021 |
| Oppland Eiendomsutvikling AS | Gjøvik | 2016 |
| Oppland Elektro AS | Østre Gausdal | 2004 |
| Oppland Hus Og Hageservice AS | Snertingdal | 2017 |
| Oppland Maler Og Gulvservice AS | Bøverbru | 2008 |
| Oppland Mur Og Flis AS | Gjøvik | 2002 |
| Oppland Oljefyringservice AS | Lillehammer | 2020 |
| Oppland Tv-Service AS | Skjåk | 2013 |
| Oppsal Bolig AS | Oslo | 2014 |
| Oppussing & Renovering AS | Rådal | 2025 |
| Oppussing & Vedlikehold AS | Sandvika | 2005 |
| Oppussing Bergen AS | Rådal | 2018 |
| Oppussing Eksperten AS | Rykkinn | 2023 |
| Oppussing Gosz AS | Fetsund | 2020 |
| Oppussing Hjem Og Hage AS | Asker | 2024 |
| Oppussing Og Byggefirma Majewski Artur Nuf | Ise | 2008 |
| Oppussing Og Sånn AS | Kvitsøy | 2016 |
| Oppussing Service AS | Gran | 2026 |
| Oppussing Åsen AS | Stavanger | 2025 |
| Oppussingseksperten AS | Oslo | 2023 |
| Oppussingtjenester Marcin Kubas | Oslo | 2007 |
| Opra Kotsøy AS | Støren | 2015 |
| Opsal Anlegg AS | Vestby | 2025 |
| Opsal Bygg Og Betong AS | Vikedal | 2019 |
| Opsal Og Fjelnset AS | Vinjeøra | 1982 |
| Opshaug & Moe Fjellsprengning AS | Stranda | 2011 |
| Opshaug AS | Stranda | 1970 |
| Opshaug Invest AS | Stranda | 2023 |
| Opstad Holding AS | Midsund | 2005 |
| Opstad Maskin AS | Midsund | 1997 |
| Opteo AS | Sandnes | 2018 |
| Optera Fiberarbeten Nuf | Se-83691 Lit | 2017 |
| Opti Construction Tomasz Bielecki | Jar | 2004 |
| Optibygg AS | Molde | 2026 |
| Optice Eco Tec AS | Kongsvinger | 2026 |
| Opticon AS | Ytre Enebakk | 2004 |
| Opticon Solutions AS | Ytre Enebakk | 2015 |
| Optielektronik T. Kowalak SP. Jawna | 61-255 Poznan | 2010 |
| Optilås AS | Ranheim | 2020 |
| Optima Bygg AB Nuf | Oslo | 2011 |
| Optimal Bygg & Betong AS | Skien | 2023 |
| Optimal Bygg AS | Vestnes | 2022 |
| Optimal Elektro AS | Oslo | 2022 |
| Optimal Elektro Holding AS | Oslo | 2022 |