A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | 1
| O'Regan's (8) | OCCMundial (5) |
| Ochsner (7) | Odgers Berndtson (6) |
| OEM Remanufacturing (5) | OfficeTeam (126) |
| Olsen Europe (9) | Olympus (15) |
| ON Semiconductor (21) | Online Business Systems (32) |
| Ontario Public Service (21) | Ontario Teachers' Pension Plan (11) |
| ontorio security training (26) | Open Text (10) |
| Optiema Inc (51) | Optima Communications (6) |
| Oracle (8) | Orckestra Conseil inc. (7) |
| Orion Project Services Ltd (10) | Osler (13) |
| OTIS (6) | Ottawa Citizen Ad (23) |
| Overwaitea Food Group (19) | Overwaitea Foods (16) |