| Norwegian Stonetech AS | Flisa | 2026 |
| Norwegian Well Control AS | Jørpeland | 2007 |
| Nos Rental AS | Sandnes | 2006 |
| Notodden Pukk AS | Notodden | 2018 |
| Nov Process & Flow Technologies AS | Fornebu | 2016 |
| Nov Wellbore Technologies Norway LLC Nuf | Stavanger | 2003 |
| Novemco AS | Oslo | 1997 |
| Nowsco Well Services GmbH | Stavanger | 1997 |
| NRC Engineering Ltd Nuf | Bh19 1Jl Dorset | 2025 |
| Nsi Technologies Inc | Us-Oklahoma 74136 | 1995 |
| Numedal Naturstein AS | Nore | 2021 |
| Numedal Pukk & Steinproduksjon AS | Uvdal | 2005 |
| Nussir ASA | Kvalsund | 1985 |
| Nye Dovreskifer AS | Dovre | 2022 |
| Nye Sulitjelma Gruver AS | Kvalsund | 2011 |
| Nøste Energi AS | Sandnes | 2004 |
| Nøtland Masseuttak AS | Vanse | 2005 |
| Ocean Flower AS | Hareid | 2019 |
| Ocean Installer AS | Stavanger | 2011 |
| Ocean Installer Holding AS | Stavanger | 2015 |
| Ocean Installer II AS | Stavanger | 2010 |
| Ocean Rig UK Ltd | Sandnes | 1999 |
| Oceaneering AS | Sandnes | 1973 |
| Oceaneering International Services Ltd | Aberdeen Ab21 0Br | 1995 |
| Ocv Recovery AS | Oslo | 2002 |
| Odal Grus AS | Skarnes | 2002 |
| Oddvar Øygard AS | Torpo | 2003 |
| Odfjell Drilling (UK) Ltd | Aberdeen Ab12 3Lg Scotland | 2005 |
| Odfjell Drilling AS | Kokstad | 2002 |
| Odfjell Invest AS | Kokstad | 2005 |
| Odfjell Invest II AS | Kokstad | 2018 |
| Odfjell Offshore Ltd Nuf | Kokstad | 2016 |
| Odfjell Operations AS | Kokstad | 1991 |
| Odfjell Technology AS | Kokstad | 2017 |
| Odfjell Terminals Global Holding AS | Bergen | 2024 |
| Odfjell Well Services AS | Kokstad | 2017 |
| Odfjell Well Services Norway AS | Tananger | 2001 |
| Odin Diving Norge AS | Oslo | 2022 |
| Oeg Offshore UK Limited | Aberdeenshire Ab51 0Qg Scotland | 2022 |
| Offcon Limited | Pembrokeshire Sa61 2Bp | 2015 |
| Offshore Commissioning Solution | West Australia 6210 | 2010 |
| Offshore Design Limited | Aberdeen Ab12 3Ax | 2012 |
| Offshore Marine Contractors Ltd | Norfolk Nr31 Onx | 2000 |
| Offshore Marine Solutions (Eastern) Limited | Lowestoft Nr32 2Hd England | 2015 |
| Offshore Resource Group AS | Randaberg | 2007 |
| Offshore Solution Partner AS | Larvik | 2015 |
| Ofoten Sand AS | Ankenes | 2023 |
| Oh Natursand AS | Indre Arna | 1983 |
| Oil States Industries (UK) Ltd | Aberdeen Ab12 3Gq Scotland | 1997 |
| Oilfield Water Services Limited | Cornwall Tr11 5Gh | 2011 |
| Ois PLC | Stavanger | 1995 |
| Okea ASA | Trondheim | 2015 |
| Oko Eiendom Da | Gol | 2008 |
| Olav Ingstadsvei 5 AS | Rud | 2006 |
| Old Kværner Invest AS | Lysaker | 1987 |
| Oljevedlikehold AS | Straumen | 1997 |
| Olle Michelsen AS | Brekkvasselv | 1977 |
| Ollendorff Holding AS | Finstadjordet | 2017 |
| Olympic Ares AS | Fosnavåg | 2015 |
| Olympic Artemis AS | Fosnavåg | 2014 |
| Olympic Challenger AS | Fosnavåg | 2018 |
| Olympic Crew Management AS | Fosnavåg | 2013 |
| Olympic Crewing AS | Fosnavåg | 2005 |
| Olympic Delta AS | Fosnavåg | 2014 |
| Olympic Electra AS | Fosnavåg | 2006 |
| Olympic Group AS | Fosnavåg | 2015 |
| Olympic Holding Chartering AS | Fosnavåg | 2010 |
| Olympic Offshore AS | Fosnavåg | 2014 |
| Olympic Offshore Wind AS | Fosnavåg | 2020 |
| Olympic Operation AS | Fosnavåg | 2013 |
| Olympic Operation II AS | Fosnavåg | 2014 |
| Olympic Services AS | Fosnavåg | 2016 |
| Olympic Ship International AS | Fosnavåg | 2015 |
| Olympic Subsea ASA | Fosnavåg | 2016 |
| Olympic Subsea Shipholding AS | Fosnavåg | 2011 |
| Olympic Taurus AS | Fosnavåg | 2014 |
| Olympic Triton II AS | Fosnavåg | 2014 |
| Oman LNG L.L.C | Bawshar/A1 Qurm | 2014 |
| Omega Subsea AS | Bergen | 2012 |
| Omega Well Intervention AS | Stavanger | 2012 |
| Oms Investering AS | Stavanger | 2024 |
| Omv Norge AS | Stavanger | 2005 |
| On & Offshore Holding AS | Kristiansand S | 2007 |
| On & Offshore Services AS | Kristiansand S | 2003 |
| One Petroleum Limited Nuf | Stavanger | 2019 |
| Onesubsea AS | Lysaker | 2022 |
| Onesubsea UK Limited | Leeds Ls10 1Sb England | 1995 |
| Online Engineering AS | Knarrevik | 2019 |
| Onsite Treatment Services AS | Tananger | 2001 |
| Opgård Maskin AS | Alta | 1990 |
| Opofte Naturstein AS | Øyestranda | 2020 |
| Oppdal Naturstein AS | Lønset | 1996 |
| Oppdal Sten AS | Oppdal | 1993 |
| Optime Subsea AS | Notodden | 2005 |
| Optimiser AS | Lepsøy | 2005 |
| Optimus Aberdeen Ltd | Aberdeen Ab15 4Al Scotland | 2005 |
| Or Råstoff AS | Straumsjøen | 2021 |
| Orca Marine Service Tech | Us-6489 No Arlington | 2015 |
| Ordalen AS | Kvaløya | 2006 |
| Org Engineering AS | Randaberg | 1999 |