| Name | Phone | Category |
| OAK TRACE | (630) 769-6200 | Retirement Community |
| OAKWOOD CONSTRUCTION SERVICES, INC. | (312) 288-9494 | Project Management |
| OFFICE 8 | (800) 672-7370 | Office Supplies |
| OFFICE FURNITURE SOLUTIONS | (630) 357-5600 | Office Furniture & Equipment |
| OFFICEMAX - CORPORATE | (630) 337-3257 | Office Supplies |
| OFFSITE IT | (312) 878-8262 | Information Technology Services |
| OLD NATIONAL | (331) 260-7313 | Banks |
| OLD SECOND NATIONAL BANK | (630) 652-2826 | Banks |
| O'LEARY LAW AND POLICY GROUP, LLC | (773) 490-6738 | |
| OMEGA RESTAURANT & BAKERY | (630) 963-0300 | Restaurants |
| OPERATION SUPPORT OUR TROOPS - AMERICA | (630) 971-1150 | Non-Profit Organizations |
| OPTIMAL PERFORMANCE MASSAGE THERAPY, PLLC | (630) 885-8002 | Massage - Therapeutic |
| ORANGETHEORY FITNESS DOWNERS GROVE - WESTMONT | (630) 326-3000 | Health Clubs |
| ORCHID HEART BOUTIQUE | (312) 933-7855 | |
| OSTERIA | (630) 434-3896 | Restaurants |
| OUTBACK STEAKHOUSE | (630) 530-0005 | Restaurants |
| OUTSOURCE IT SOLUTIONS GROUP | (630) 701-3393 | |
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