| Otis | 2730 Ballerup | 2005 |
| Otis AS | Oslo | 1973 |
| Oto Anlegg AS | Sel | 2014 |
| Oto Eiendom AS | Lillesand | 2000 |
| Otr Maskin & Transport AS | Risøyhamn | 2022 |
| Otraks D.O.O. Srbac | Ba-78420 Srbac | 2023 |
| Otraks Ltd | Ge-0800 Akhaltsikhe | 2023 |
| Otraks, D.O.O. Slovenia | Si-1262 Dol Pri Ljubljani | 2023 |
| Otravegen Prosjekt AS | Hovden I Setesdal | 2021 |
| Ott Service AS | Øvre Årdal | 2021 |
| Otta Entreprenør AS | Otta | 2021 |
| Ottadalen Anlegg AS | Vågå | 2020 |
| Ottadalen Bygg AS | Otta | 2019 |
| Ottadalen Elektro AS | Vågå | 2018 |
| Ottadalen Holding AS | Otta | 2020 |
| Ottar Augdal AS | Heimdal | 1986 |
| Ottar Berg Holding AS | Vassenden | 2004 |
| Ottar Bergersen & Sønner AS | Sortland | 2021 |
| Ottar Bergersen Holding AS | Straumsjøen | 2021 |
| Ottar Dvergsdal AS | Skei I Jølster | 2025 |
| Ottar Myklebust Anlegg AS | Ålesund | 2025 |
| Ottar Skjold Maskin AS | Øvre Vats | 2003 |
| Ottars Gate 1 AS | Narvik | 2011 |
| Otterdal Vatne AS | Ørsta | 2024 |
| Otterstad Maskin & Gartnertjenester AS | Sokna | 2022 |
| Ottesen Bygg AS | Oslo | 2015 |
| Ottesen Bygningsfirma AS | Hamarvik | 1993 |
| Ottesen Maskin & Transport AS | Tromsø | 2008 |
| Ottestad Rørservice AS | Ottestad | 2010 |
| Otto Blichfeldt AS | Klokkarstua | 2013 |
| Otto Junker GmbH | 52152 Simmerath-Lammersdorf | 2003 |
| Otto Moen Installasjon AS | Otta | 2018 |
| Ottosen Graving Og Transport AS | Næroset | 2019 |
| Ou Andi Retro Nuf | Ee-10616 Tallinn | 2025 |
| Ou Saare Tørv | Saaremaa 93301 | 2015 |
| Ouissar Blikkenslager AS | Lørenskog | 2025 |
| Oulun Sähkö-Power OY Nuf | Fi-90620 Oulu | 2026 |
| Ous Holding AS | Stranda | 2005 |
| Ov Entreprenør AS | Rasta | 2024 |
| Ovacon AB Nuf | Se-619 33 Trosa | 2025 |
| Ove Dahl Holding AS | Larvik | 2009 |
| Ove Gunnar Viken AS | Bagn | 2023 |
| Ove Halvorsen AS | Algarheim | 2013 |
| Ove K Helland AS | Stord | 2015 |
| Ove Mur Og Flis AS | Kokstad | 2019 |
| Ove Skår AS | Sarpsborg | 1989 |
| Ove Skår Gruppen AS | Sarpsborg | 2017 |
| Ove's Rør & Mek. AS | Helleland | 2008 |
| Oveido Oü | 10921 Tallin | 2013 |
| Overen Maskin Og Utemiljø AS | Brandbu | 2024 |
| Overland Maskin AS | Grimstad | 2002 |
| Overvik Maskin AS | Trondheim | 2014 |
| Overvåkningseksperten AS | Ski | 2025 |
| Oves Byggteknikk AS | Sande I Vestfold | 2013 |
| Oves Sandblåsing Sprøytemaling Da | Dalekvam | 1991 |
| Ovi Bygg AS | Nøtterøy | 2024 |
| Ovimar Byggeservice AS | Frekhaug | 2025 |
| Ovimatari Investering AS | Lyngdal | 1991 |
| Ovn Og Peis Montering AS | Kopervik | 2021 |
| Ovn Og Pipe AS | Åmli | 2022 |
| Ovnsfag Da | Godvik | 2010 |
| Ovnskroken AS | Ørje | 2012 |
| Ovnsmontering AS | Fiskum | 2024 |
| Ovnsmontøren AS | Birkeland | 2014 |
| Ovo AS | Ise | 1989 |
| Ovs AS | Spydeberg | 2017 |
| Ovs SA | Oslo | 2022 |
| Owesen AS | Selje | 2023 |
| Owest Bygg | Ee-93810 Saare Maakond | 2018 |
| Owl Contractor Services Ltd | Newcastle Upon Tyne Ne1 8Jw | 2021 |
| Owren Gastech AS | Vingrom | 2023 |
| Owrum Bygg Og Blikk AS | Spydeberg | 2019 |
| OY at Install AB | Fi-85100 Kalajoki | 2021 |
| OY C.E. Lindgren AB | Fi-06450 Porvoo | 2021 |
| OY Christeyns Nordic AB | Fi-20250 Turku | 2025 |
| OY Forssa-Engineering AB | Fi-20780 Kaarina | 2015 |
| OY Temacon Ltd | 60100 Seinäjoki | 2010 |
| OY Tricolme Ltd | Fi-20300 Åbo | 2018 |
| Ozog Service AS | Aurskog | 2024 |
| Ozon Kube AS | Nesbru | 2022 |
| Odd-Petter Hosen AS | Stokkøy | 1988 |
| Olaf E Eriksen AS Entreprenørforretning | Oslo | 1931 |
| Oslo Elektro - Service A/S | Oslo | 1984 |
| Oü Alevikinnisvara Nuf | Ee-11314 Tallinn | 2024 |
| Oü Builder & Joilie | Søreidgrend | 2008 |
| Oü Enterfan | 61001 Tartumaa | 2007 |
| Oü Ew Production Nuf | 11711 Tallinn | 2012 |
| Oü Hegaz El Batrawy Nuf | 10134 Tallin | 2010 |
| Oü Kondra Sva | 44314 Rakvere | 2011 |
| Oü Kvnplu Nuf | Ee-50411 Tartu, Tartu Maakond | 2024 |
| Oü Maleva Betoon Nuf | Ee-75512 Hajumaa | 2026 |
| Oü Maurice Bygg | Tomrefjord | 2012 |
| Oü Merlinder Nuf | 11913 Tallin | 2006 |
| Oü Otterm | 51011 Tartu | 2009 |
| Oü Royalstone Grupp | Ee-72210 Turi | 2009 |
| Oü Spex Nuf | Hammerfest | 2008 |
| Oü Weldpipemetall Nuf | Ee-13816 Tallinn | 2025 |
| P & a Eiendom AS | Alta | 2012 |
| P & a Holding Alta AS | Alta | 2020 |
| P & T Services Spolka Cywilna | 43-600 Jaworzno | 2005 |