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| O'Regan's (8) | O'Regan's Automotive Group (6) |
| OCCMundial (6) | Ochsner (7) |
| Odgers Berndtson (7) | OEM Remanufacturing (5) |
| OfficeTeam (126) | Olsen Europe (9) |
| Olympus (17) | ON Semiconductor (22) |
| Online Business Systems (31) | Ontario Institute for Cancer Research (5) |
| Ontario Public Service (22) | Ontario Teachers' Pension Plan (11) |
| ontorio security training (25) | Open Text (10) |
| Optiema Inc (51) | Optima Communications (6) |
| Oracle (8) | Orckestra Conseil inc. (7) |
| Orion Project Services Ltd (9) | Osler (13) |
| OTIS (6) | Ottawa Citizen Ad (21) |
| Overwaitea Food Group (25) | Overwaitea Foods (15) |