- One to One Personnel
- One YMCA
- OneCall24 Limited
- Onecare-uk.com Ltd
- Oneill & Brennan
- Open Spaces IT Recruitment
- Open Systems Recruitment
- OpenJobs
- OPKiT Ltd
- Optima Site Solutions
- Optimum Recruit Solutions LLP
- Optimum Security Supplies Ltd
- Options Employment (Midlands & West) Ltd
- Options Recruitment (Scotland) Ltd.
- Opulent Catering
- Opus People Solutions
- Oradeo Recruitment
- Orange Resources Recruitment Agency
- OrangeMarble LImited
- Orbis Consultants
- Origin Multilingual
- Orion Electrotech Ltd
- OSC Telecoms Consultancy Ltd
- Osteotec Limited
- Outcomes First Group
- Outset Uk
- Outset UK Limited
- Outsource UK Limited
- Outsource UK Limited
- Ovarian Cancer Action