| Old Crookhey Limited | Lancaster | 2006 |
| Old Hall Farm Barns Management Company Limited | Norwich | 2006 |
| Old Mill Environmental Recycling Limited | Vale Of Glamorgan | 2006 |
| Olde English Building Materials Limited | Preston | 1990 |
| Oldfields Reclamation Ltd | Brierley Hill | 2008 |
| Oldham Recycling Centre Ltd | Manchester | 2018 |
| Oleco VN Ltd | London | 2021 |
| Oleo Environmental Limited | Doncaster | 2020 |
| Oleo Recycling Limited | Doncaster | 2020 |
| Olive Engineering Services Limited | Bray On Thames | 2016 |
| Oliver Farm Recycling Ltd | Woodbridge | 1972 |
| Oliver Grace Ltd | Burntwood | 2015 |
| Olleca Karlos Limited | London | 2025 |
| Olleca Limited | London | 2025 |
| Ollybogs Services Ltd | London | 2025 |
| OLS Draincare Ltd | Enniskillen | 2024 |
| Olus Biomass Limited | Hove | 2012 |
| Olus Environmental Limited | Hove | 1983 |
| OM Dhriti Enterprise Ltd | Leicester | 2023 |
| OM Skips & Recycling Limited | London | 2008 |
| Omar Homes Limited | Manchester | 1965 |
| Omdm Ltd | Sidcup | 2023 |
| Omega Developments (Midlands) Ltd | Tamworth | 2022 |
| Omega Environmental Services Limited | Hitchin | 2012 |
| Omnex Water Ltd | London | 2026 |
| Omni Recycled Materials Limited | Tunbridge Wells | 2012 |
| Omni Recycling Limited | Tunbridge Wells | 2015 |
| OMNI360 Ltd | Salisbury | 2024 |
| Omnisol Limited | London | 2013 |
| Omniwater Group Ltd | London | 2026 |
| Omniwater Ltd | London | 2026 |
| Omwater Systems Limited | Bristol | 2020 |
| ON & Gone Waste Management Services Ltd | London | 2021 |
| ON and Gone Forever Ltd | Glasgow | 2025 |
| ON Demand Services Group Ltd | Neston | 2024 |
| ON Site Project Services (Central) Ltd | Dunfermline | 2023 |
| ON Site Project Services Ltd | Motherwell | 2014 |
| ON TAP (South West) Limited | Somerton | 1998 |
| ON the DOT Limited | | 1974 |
| ON the Go Recycling Limited | Newcastle | 2010 |
| ON the JOB Waste Management Ltd | Waltham Cross | 2025 |
| ON the Move Fleet Ltd | London | 2025 |
| ON the Trash Waste Removal Ltd | Blyth | 2024 |
| ON Time Shred Limited | Woodford Green | 2017 |
| Onbits Global Limited | Harrow | 2022 |
| Onboard Waste Services Limited | London | 2024 |
| One Birmingham Motor Works Ltd | Birmingham | 2013 |
| One Business Water Ltd | Leicester | 2022 |
| One Caal Rubbish Removal Limited | Dagenham | 2024 |
| One Call Waste Ltd | Leeds | 2009 |
| One Clear (Birmingham) Ltd | Birmingham | 2025 |
| One Clear (Leicester) Limited | Leicester | 2025 |
| One Clear (Manchester) Limited | Manchester | 2025 |
| One Clear Choice Ltd | Mildenhall | 2025 |
| One Clear Ltd | Coventry | 2023 |
| One Earth Consultation Ltd | London | 2020 |
| One Group Solutions Ltd | Marlow | 2020 |
| One Stop Commerce Ltd | Huddersfield | 2022 |
| One Stop Inspector Limited | Runcorn | 2016 |
| One Stop Procurement Ltd | London | 2026 |
| One Stop Professional Building Contractors Ltd | Sheffield | 2018 |
| One Stop Recycling Limited | Birmingham | 2000 |
| One Stop Waste Ltd | London | 2025 |
| One Stop Waste Solutions Limited | Aberdeen | 2006 |
| One Vision Construction Ltd | Wakefield | 2019 |
| One Waste Clearance Ltd | Mitcham | 2014 |
| One Waste London Limited | Sutton | 2019 |
| One Waste Ltd | Sutton | 2021 |
| One Way Clearance Ltd | Gravesend | 2024 |
| One World Environmental Ltd | Belfast | 2021 |
| One-Time Solutions Ltd | Northampton | 2021 |
| Onea Metals Ltd | London | 2025 |
| Oneflow Drainage Ltd | Bournemouth | 2024 |
| Oneforty Services Limited | London | 2024 |
| Oneofficeworld-Lon Ltd | London | 2025 |
| ONEZERO8 Rebound Ltd | London | 2022 |
| ONG Rainmadagascar CIC | London | 2024 |
| Onguard Waste Removal Logistics Ltd | Waltham Cross | 2025 |
| Online Environmental Limited | Stoke-On-Trent | 2025 |
| Online Waste Services Ltd | Mansfield | 2018 |
| Only PAY What IT Weighs Limited | Dartford | 2023 |
| Only Rods and Hoses Ltd. | Weybridge | 2013 |
| Only Talking Rubbish Ltd | London | 2025 |
| Only Waste Arjan Ltd | Palmers Green | 2021 |
| Only Waste Ltd | Glasgow | 2022 |
| Onnu Green Hub Ltd | Bury St. Edmunds | 2024 |
| Onpoint Home Solutions Ltd | London | 2025 |
| Onsite Management Construction Services Limited | Southampton | 1989 |
| Onward Waste Limited | Sandown | 2020 |
| Onwater Limited | Birmingham | 2024 |
| Onyx Business Solutions Ltd | Manchester | 2023 |
| Onyx Recycling Ltd | Leeds | 2017 |
| Onyx Waste Ltd | Manchester | 2020 |
| Onyx Water Treatment Limited | Herne Bay | 2017 |
| Ootnoo Ltd | Edinburgh | 2016 |
| Ooymus Ltd | London | 2018 |
| Opal Water Limited | Sunbury-On-Thames | 2025 |
| Opalside Ltd | Woking | 2024 |
| Open Door Furniture Recycling Limited | Burnley | 2004 |
| Open Space Regeneration Limited | London | 2017 |